BMW explicitly claims it is not necessary to change lifetime transmission oil at any point through the service life of the transmission. ZF, the transmission manufacturer, on the other hand, recommends the oil change at 50,000 – 90,000 miles intervals.
So, who should you trust?
When I first started working on BMWs back in 1996, BMW introduced maintenance-free automatic transmissions supplied by ZF, General Motors, and Jatco (Japan only) to reduce maintenance costs. I believe that BMW mechanics back then had a lot more faith in BMW, thus this was a no-brainer for us.
But transmission rebuilders never bought this story. They claimed, although each of them differently, that you must change the transmission oil regularly. The transmission manufacturers like GM and ZF also stand by their side, but with slightly longer interval recommendations.
Twenty-six years later and nothing changed. BMW still claims that transmission oil needs never be changed, although ZF, GM, and Aisin claim that it should be replaced every 20,000 to 90,000 miles, depending on how you drive and who you ask.
While both sides of the debate hold some truth and logic, neither is entirely correct.
Let’s open this can of worms.
The arguments for regular oil changes are strong: transmission oil oxidizes, lubrication additives’ molecules dissolve, friction modifiers lose their properties, the water condensates inside transmission housing and enters the oil, the oil becomes contaminated with abrasive debris, the filter clogs, and so on… It’s an easy sell.
But, as it turned out, almost three decades of maintenance-free BMW transmission thought us that transmission oil doesn’t oxidize that easily, the water doesn’t condensate in the transmission housing, the oil contamination actually doesn’t affect shift quality, and while lubrication properties are lost for sure, they are good enough to do their job.
While, without a doubt, changing transmission oil can only be a big positive and can prolong the transmission’s ‘service life’ expiration date, at least from my experience, the transmission that never had an oil change will usually outlast its regularly maintained counterpart.
If you’re wondering how that could possibly happen, it’s actually very easy.
But before we move on, let’s take a look at how transmissions break.
What Kills Automatic Transmissions?
The biggest enemy of any automatic transmission is the vicious cycle of oil overheating. When transmission oil reaches its flash point (about 400°F/206°C) it starts to burn and loses viscosity and lubricating qualities, which in turn produces even more heat and, eventually, total failure. It’s the cycle you want to avoid.
And when the oil is burned there’s no point in changing it. Burned transmission oil equals burned clutch packs.
Now, let’s talk about what causes overheating in the first place.
Lack of Transmission Oil
If you want to overheat and destroy your BMW’s automatic transmission, lengthy idling in traffic on a hot day, 100-mile top speed drive, weekly track days, ten quarter-mile drag races in a row, or dragging a 30-foot boat uphill at full throttle are all excellent options.
However, in my experience, the most prevalent offender is a lack of transmission oil. Because there’s less oil circulating the system, the remaining oil is put under more stress and heats up faster.
If you’re thinking that 5 or 10% of volume loss is not that significant, think again. According to Arheniuss’ equation, for every 18°F increase in oil temperature, the oil life is cut in half. That’s why transmissions that lack a bit of oil will usually drive for months, sometimes even a couple of years before they fail.
There are two causes for the lack of oil: a leak or someone didn’t put in enough oil.
Now, here’s the intriguing part.
When there is a leak, even a large one, like a leaking oil cooler line’s o-rings, the transmission will usually work perfectly after repair and refill for years, as if nothing happened.
When there is no leak, however, the transmission is usually irreparably ruined.
Leaks, in my opinion, are easier to detect than a shortage of oil. Small leaks are normally discovered during routine maintenance, whereas massive leaks cause acute symptoms and motivate the owner to act immediately.
On the other hand, the lack of transmission oil without an oil leak is hard to detect and BMW’s maintenance schedule does not include ‘check transmission oil level’ at any point. However, it does include a visual check for leaks.
These ‘small losses’ are actually the most dangerous silent killers, and just like high blood pressure, they may not show any immediate symptoms.
In conclusion, incorrect oil refill procedures are the most common killer of BMW automatic transmissions.
Ok, let’s move on to another common fault.
Wrong Transmission Oil
Most modern automatic transmission oils are developed alongside the transmission. Chrysler, I believe, was the first to do it in the early 1990s with their ATF+3. The additive compound was specially formulated for friction disc (clutch) material.
If you use a different oil, for example, Dexron or Mercon, you’d burn both the oil and the clutch packs, again because of excessive heat.
For a wet clutch in an automatic transmission to function properly and smoothly, friction in the clutch contact must be high throughout the sliding speed range and increase with sliding speed. The friction disc material and morphology, as well as the automatic transmission oil, are precisely designed to achieve this.
Despite this known fact, filling transmissions with the wrong oil has been happening for decades. If you’re wondering why – it’s actually pretty easy.
If you go aftermarket, there are many ways to buy the incorrect oil. Mislabeled, misleadingly labeled, and fraudulently bottled transmission oils (as another product) is an ongoing problem that lasts for decades. And since it’s so profitable I don’t see it going away in the foreseeable future.
The “accidental” mistakes also happen because many shops use the same machine or pump to fill different types of transmission oils. Even a half quart of wrong oil left in the pump can cause irreversible damage down the line.
So, if you ask me, look no further than OE BMW and ZF Lifeguard transmission oils from a reputable supplier.
And don’t forget to fill the transmission with a dedicated or clean pump.
If you’re wondering if there’s a difference between the BMW and ZF oil, besides the price, I really don’t know. If we follow the logic of other aftermarket parts, like tires, filters, or brake pads, there could be a difference, but I don’t want to open another can of worms. All I can tell you is that I use ZF Lifeguard for my cars and I can’t tell the difference.
Shift Management Fault
Issues happen when you ignore the obvious symptoms for too long. A few quirks here and there are not problematic. They are mostly caused by adaptations and a driving style learning function.
But when you are driving for months with obvious shudders, harsh shifts, RPM fluctuation at a constant speed, and so on… you’re calling for disaster.
The ‘Transmission Malfunction’ warning message will usually come when it’s already too late.
When there’s a fault in the shift management system, caused by faulty mechatronics’ pressure regulators, separator (gasket) cracks, or clutch bearings, the clutch-to-clutch overlapping times are mismatched.
The precondition for a smooth shift is precisely timed pressure overlap between the oncoming and off-going clutch. When the timing is right, there’s no power interruption and you get a smooth shift.
If, for example, one of the mechatronics’ pressure regulators lags or leaks, the transition from the off-going to the on-coming clutch will not happen timely and clutch packs and oil will start to heat up and burn.
Now that we covered the most common problems, let’s dig a bit deeper into the world of automatic transmission oils.
Can Automatic Transmission Oil Really Last a Lifetime?
The transmission oil can really last for a lifetime. I’ve seen it many times in the flesh. 300,000 or even 400,000 miles on a factory fill is more than reasonable service life in my opinion.
To understand how this became possible, let’s take a journey on transmission oil development.
The Evolution of Automatic Transmission Oil
Back in the 1950s and 1960s, most cars arrived at junkyards with fully functioning automatic transmissions. Mechanics never changed the transmission oil until a leaking gasket or seal, and the 100,000-mile oil change interval was the norm.
In 1967 Ford released the Type-F fluid which called for the creation of a “lifetime” fluid that would never need to be changed. This was the first of many ‘lifetime’ oils that other companies have followed.
The times were good. Automatic transmission reliability grew alongside the US 1960s economic boom.
Then everything went wrong.
In the 1970s, two major events sent automatic transmissions to the dark ages: the prohibition on whale hunting and the Clean Air Act.
In 1971 the sperm whale was put on the endangered species list leaving the automatic transmission oils without the main ingredient: sperm whale oil.
Without Mobi Dick’s spermatozoid oil, the automatic transmission started to fall apart. Prior to 1972, fewer than one million transmissions broke each year; however, in the absence of sperm whale oil, transmission breakdowns surged to more than eight million by 1975.
Moby Dick’s oil was chosen for its superior lubricating and anti-oxidation qualities. Without it, the oil oxidation was so severe that it corroded cooling radiators, and when coolant and transmission oil mixed, catastrophic failure occurred quickly.
The Clean Air Act put the final nail in the coffin. The increased emission restriction required engines to operate at higher temperatures. So the increased coolant temperature increased transmission temperatures even higher.
Only with the temperature increase can we forget about the lifetime oil, or even the 100,000 oil change interval. Without the working alternative for whale oil, we are talking about intervals of 15,000 – 20,000 miles under normal operating conditions.
The real alternative for sperm whale oil came only in the late 1980s in the form of another natural ingredient: jojoba oil.
Jojoba oil is actually an ester, just like whale oil, and has almost exact qualities. But the alternative didn’t last for long. By 1992 it became hard to source the required amounts of jojoba oil.
Around that time, a company called International Lubricants developed and patented the first and only workable synthetic Liquid Wax Ester (LXE) as a replacement for both sperm whale oil and jojoba oil. It was sold as a supplemental additive to transmission oil and the reaction of most mechanics back then was about the same as to snake oil.
But then, in 1993, they got the official ZF’s approval and fixed problems in ZF transmissions used by BMW and others. LUBEGARD ATF Protectant sold over one million bottles in 1993.
We can only speculate that this synthetic wax was the missing ingredient that enabled BMW to ‘convert’ all automatic transmissions to maintenance-free by 1996.
Whatever happened, the automatic transmissions were back on track.
And, with the help of a few more breakthroughs, the future of automatics appeared bright again.
Why Does Transmission Oil Need to Be Changed?
Oxidation of automatic transmission oils is the main reason for recommending periodic oil changes. Oxidation deteriorates the frictional properties of the oil, causing poor shift performance, and eventually clutch failure.
Oxidation also attacks metal parts and seals and forms sludge and varnish deposits which can stick pressure regulators and plug oil filters.
So, what causes oil oxidation?
Overheating and transmission inhaling fresh air – as the oil temperature changes so do the oil volume. When the temperature increases the gasses are vented into the atmosphere. As the oil cools down, the pressure transforms into a vacuum, and the transmission inhales fresh air.
To reduce transmission breathing, and therefore reduce oil oxidation, a one-way check valve called the Transmission Air Breathing Suppressor valve (TABS) was designed to replace the conventional vent.
If you ever saw the ZF6HP or ZF8HP transmission in the flesh, that’s the tiny blue cap on top of the transmission.
When a particular gas pressure is exceeded, the TABS valve allows gas in the transmission to exhaust via the valve into the atmosphere, but it prevents airflow into the transmission when a vacuum is generated.
Although the transmission housing isn’t 100% sealed, and some air enters the transmission during each cool-down cycle, the amount of oxidation is insignificant.
With air put out of the equation, there’s only the heat problem left.
As it turns out, the progress in transmission design and oil development eliminated the need for an oil change until a repair, leak, or overheating.
The last one is the most problematic, as there aren’t many ways to know if the oil was overheated. You can take a sample, and if it smells burned – you have a bigger problem than deciding whether should you change the oil. If, on the other hand, it doesn’t smell burned, you can send your sample to the laboratory for analysis.
Thanks to Blackstone Labs, the process is rather simple.
We only have the problem of burned oil with ultra-high-quality oils and modern transmissions.
How Long BMW Automatic Transmissions Last
Predicting the life expiration date of your transmission is akin to predicting the stock market. You will be able to predict more accurately if you understand the system and have vast experience with it. But, let’s try to keep it simple.
In 2014, a production version of the ZF8HP45 transmission with just software changes completed its maiden racing season. It was raced in the BMW M235i Racing Cup for private teams as well as a BMW junior program.
The basic transmission foundation, designed to handle up to 450 Nm of torque and a top speed of 7200 rpm, proved to be powerful and sturdy enough to meet all longitudinal and lateral dynamic needs. However, with a service life of about 20,000 miles in race use.
When it comes to race usage, 20,000 miles seems adequate.
On the other end of the scale, a 300,000-mile 7-Series on a factory fill is nothing uncommon, but it’s not the norm.
Most modern BMW automatic transmissions (ZF6HP and ZF8HP) typically last between 200,000 and 250,000 miles – with or without oil changes.
In conclusion, the length of transmission service life is mostly affected by your driving style and operating conditions.
To Change The Oil, or Not to Change The Oil?
Me? I never changed the automatic transmission oil in any of my BMWs until there was a leak, and most of them never leaked. At first, I blindly followed what BMW recommended, but over time, I’ve become convinced that it really works.
But I can’t recommend my customers not to replace the transmission oil. It’s my choice because I have experience and understand how the system works. Add to that the fact that I’m cheap and I don’t like repairing my own cars, so I take precautions where I can – while driving.
When I’m towing a boat, I’ll drive off gently while thinking about how much a transmission repair would cost, both in my time and money and then I’ll engage the cruise control.
BMW transmissions are filled with lifetime oil since 1996, and oil changes are generally only required after repair. In my opinion, this will work as long as you drive moderately – or never overheat the oil.
However, because numerous factors influence transmission service life in individual operations, I recommend changing the oil after 100,000 miles, or at least once in the lifetime.
In operating conditions with high temperatures and loads, or with unknown previous (ab)use, shortening the interval to 60,000 miles makes perfect sense.
Unfortunately, there is a good chance that you’ll end up with less oil than you had before the change with a bit of the wrong oil in the mix.
In conclusion, changing the lifetime transmission oil after 60,000 to 100,000 miles makes sense, but only if done correctly and with the proper oil.