[JustinHEMI]

Now that people are getting their break in done. What is the part number of the fluid they put in your rear end? I'm concerned they got mine wrong, but they insist that they didn't.

83222447362

[JustinHEMI]

Here's a thread in the G80 forums where someone cites the same part number. Starting to feel better.

https://g80.bimmerpost.com/forums/sh....php?t=1990209

[InspectorSeb]

That matches the part # they used for mine

[JustinHEMI]

Ok cool. Another person on Facebook posted it as well.

I'm satisfied.

[CarPoor]

I’ve got this ready to go in my 240 as well. Went through a fairly exhaustive effort to back into the spec’d fluid. Will probably step up viscosity at around 10K miles though once I have the fluid ripped apart in my company’s lab unless that reveals something beyond lsd add pack and lower viscosity.

[RockCrusher]

Originally Posted by CarPoor

I’ve got this ready to go in my 240 as well. Went through a fairly exhaustive effort to back into the spec’d fluid. Will probably step up viscosity at around 10K miles though once I have the fluid ripped apart in my company’s lab unless that reveals something beyond lsd add pack and lower viscosity.

More viscosity? Why?

More viscosity will make the fluid run hotter and could lead to a break down in lubrication.

[CarPoor]

Originally Posted by RockCrusher

More viscosity? Why?

More viscosity will make the fluid run hotter and could lead to a break down in lubrication.

That’s somewhat farcical actually. It’s true it will run hotter, but not materially unless there is a substantial increase in viscosity and that’s assuming the clearances have been revised for the lower viscosity. Operating conditions such as track use could also impact this. So, there’s an equilibrium point. From what I’m being told 75w110 is the sweet spot. Mine may see track time if my 8 goes down, but I’ll probably not go higher than 75w90 and likely just 75w85. Same for the front.

[RockCrusher]

Originally Posted by CarPoor

That’s somewhat farcical actually. It’s true it will run hotter, but not materially unless there is a substantial increase in viscosity and that’s assuming the clearances have been revised for the lower viscosity. Operating conditions such as track use could also impact this. So, there’s an equilibrium point. From what I’m being told 75w110 is the sweet spot. Mine may see track time if my 8 goes down, but I’ll probably not go higher than 75w90 and likely just 75w85. Same for the front.

Farcical yet true?... Ok.

The viscosity is selected for the clearances and load. No reason to up the viscosity or lower it without a corresponding change in clearance. The load won't change at least its maximum won't change as long as the engine remains stock.

If you are truly concerned about the effects of track use on the diff consider a diff fluid cooler. Keeping the fluid hot but not letting it get too hot will do more to ensure long and trouble free service life from the diff than playing with diff fluid viscosity.

[CarPoor]

Originally Posted by RockCrusher

Farcical yet true?... Ok.

The viscosity is selected for the clearances and load. No reason to up the viscosity or lower it without a corresponding change in clearance. The load won't change at least its maximum won't change as long as the engine remains stock.

If you are truly concerned about the effects of track use on the diff consider a diff fluid cooler. Keeping the fluid hot but not letting it get too hot will do more to ensure long and trouble free service life from the diff than playing with diff fluid viscosity.

No, not true from a materiality standpoint.

Got any evidence to reduced clearances? And before you stick your neck out there just know that a 75w90/110/140 will get through said clearances just as quickly as the OEM 75w80. If not we’d all have issues in winter.

Why risk an aftermarket cooler that could fail when upping the viscosity will more than suffice and without appreciably elevating temperatures? My track car RX8 actually saw the manual transmission temperatures drop from 250F to 220F average when I went from 75w90 to 75w110. My rear differential did similar dropping from 280F to 255F average going from 75w90 to 75w140. Been it like this for 11 years now.

[RockCrusher]

Originally Posted by CarPoor

No, not true from a materiality standpoint.

Got any evidence to reduced clearances? And before you stick your neck out there just know that a 75w90/110/140 will get through said clearances just as quickly as the OEM 75w80. If not we’d all have issues in winter.

Why risk an aftermarket cooler that could fail when upping the viscosity will more than suffice and without appreciably elevating temperatures? My track car RX8 actually saw the manual transmission temperatures drop from 250F to 220F average when I went from 75w90 to 75w110. My rear differential did similar dropping from 280F to 255F average going from 75w90 to 75w140. Been it like this for 11 years now.

Did not say the clearances changed. Just the opposite. They (obviously) stay the same.

The diff cooler does bring with it a slight risk of failure but for track use with elevated operating temperatures the benefits out weigh the risks.

Also, I note this risk concern hasn't prevented auto makers from fitting coolers for the more expensive (at least automatic) transmission and the much more expensive engine.

[CarPoor]

For anyone interested here is a VOA for the G4 fluid. Iron and insolubles are high for a VOA, but I was meticulous collecting the sample. Maybe Blackstone's equipment was dirty. Viscosity is closer to that of a DCT fluid but is within range of a 75w80. Addpack is typical of Hypoid LSD fluid and TAN is probably high from the high phosphorus which is likely a phosphorus ester.