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Why use FD Oil with different Viscosity rating?


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I was looking at the manual for the RT. BMW recommends Castrol SAF-XO for the final drive oil.


From what I can find on their not-so-easy-to-use website, castrol lists XO as 75W-90 and XJ as 75W-140. I noticed a lot of people here are using the Valvoline syn 75-140. Is there an advantage to using a different viscosity than recommended?


I usually try to stick to the recommended spec’s, for future potential warranty issues, but could see going beyond the spec, if there was some advantage.



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Since the oil in the transmission will never see the high temperatures that it would in the engine, the 140 rating in 75W-140 is of little consequence. Either oil will have the necessary viscosity to properly function in the gearbox. One less thing to worry about.

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Since the question is about final drives rather than transmissions, the question of smoother shifting is moot.


As to why, many of us prefer to use synthetic oils in this application and often, our chosen brand may not offer the synthetic in the lighter single or multi-viscosity type oil. You only can get the 75w140.


I wouldn't hesitate to use either and, so long as the oil is changed on schedule, not to worry about it.


I have a 5 gallon pail of gear oil in the back corner of my garage that I have been using up for years. I can't even remember the brand. I use it for chain lube as well.

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The final drive will run around 60 degrees C whereas the transmission will run around 110 degrees C. I measured these numbers with a 25C ambient. Based on this temperature data, a 90wt in the final drive will be more viscous than a 140wt in the tranny at operating temperature.


Next time you go for a long ride, stop and touch the final drive and then touch the transmission (be careful).

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Thanks for the replies. I changed the FD oil this afternoon, and had no problems.


Thanks to JimVonBaden and 1bmwfan for posting earlier pics and procedures!!


It was easy to do, but would have been a lot easier if they just would have put in a drain plug! I ended up using a syringe from the autoparts store (for mixing 2 stroke oil) and small tubing. I could fit the tubing in and down, and it kept the fluid from coming back out while filling.


Thanks Again!

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