Our readers often ask, "Should I change my BMW transmission oil? I've been told that it should last a lifetime". Before we answer this question, let's first understand more about the ZF 8HP transmission found in most recent BMWs.
8HP is ZF Friedrichshafen AG's trademark name for its eight-speed automatic transmission models (8-speed transmission with Hydraulic converter and Planetary gearsets) for longitudinal engine applications, designed and built by ZF's subsidiary in Saarbrücken. It had its debut in the BMW 7 Series (F01) 760Li saloon fitted with the V12 engine, and since then each new BMW model in all Series down to the 1 Series in rear-wheel-drive and all-wheel-drive versions have been equipped with it.
One of its main aims is to improve vehicle fuel economy, and it can achieve an 11% saving compared to the ZF 6-speed transmission and 14% versus modern 5-speed transmissions. Due to changes in internal design, the shift times have reduced to 200 milliseconds; additionally, the unit brings the ability to shift in a non-sequential manner - going from gear 8 to gear 2 in extreme situations. In the 8HP70 version, it has a torque handling limit of 700 newton metres, and weighs 87 kilograms. [1]
To answer the question if transmission oil is supposed to last 'forever', let's have a look at a section of the ZF published data sheet below[2]:
"... automatic transmissions are filled maintenance-free with specially developed partially synthetic ATF oils. Maintenance-free fills are intended for normal operating conditions. Especially driving at very high operating temperatures can result in accelerated aging or increased wear of ATF oils. It is recommended, in the event of severe operating conditions, such as:
frequent highway driving in top speed range,
offensive, sporty driving style,
frequent trailer operation,
being above average, oil purification (oil change) on automatic transmissions is recommended between 80,000 km and 120,000 km, or 8 years, depending on the load."
With the statement above, we can conclude and recommend that transmission oil change for the ZF 8HP should be done based on the mileage of years of vehicle ownership.
Now that we've established that it is important to change the transmission oil, let's check the recommended procedure published by ZF.
Before the work starts, the following items will be required:
New ZF Gear oil 8HP
New ZF Pan (with filter)
New screws and retainer screws
The gear oil best to changed when the car is able to be jacked up because there is need to remove the gearbox cover and all the activities needs to be performed at the bottom of the car. So best place to get this done is at the workshop where they have proper tools, knowledge and skills to handle it in a safe manner.
The following sections explains and illustrates the oil change procedure.
1. First, raise the car to remove the gearbox cover. This allows access to the transmission pan.
2. Next, the drain plug is removed to drain the used gear oil.
3. Drain the oil.
4. Remove all pan screws and filler screws.
5. Remove the pan and clean the solenoid and mechatronic thoroughly and ensure each screws are tight. Check for foreign objects.
6. Install the new pan and tighten the screws with a torque wrench(8nm).
7. It is important to replace all existing screws with a new set. Humidity and water causes the screws to rust over time.
8. Fill the gear oil with a pump. Fill it to the brim(until the oil spills out). It takes approximately 4 liters of oil for the first filling.
9. Fire up the engine and shift to each gear and hold each gear for 10 seconds. Warm gearbox to around 40-50 degrees celsius.
10. Refill the oil again to the brim until it overfills and secure the filler screws. Reinstall the cover and the ATF change job is done.
Now that you understand the reason and the procedure behind a ZF 8HP transmission oil change, you can make an informed decision to change(or not to change) your ATF oil the next time you service your car. Happy driving!
References: [1] https://en.wikipedia.org/wiki/ZF_8HP_transmission