951pcarguy

I changed my gear oil to use Redline MT-90, one of the recommended synthetic GL-4 type gear oils for my 87 951 with Limited Slip, and now I'm getting a pretty noticeable whine from the transmission when in gear above 50mph.

Anyone else have this same result? I'm thinking about going back to synthetic GL-5 to see if it makes a difference.

I also noticed that one of the half shafts is clunking when I move it back and forth, so I was wondering if that might be the culprit as well.

KevinGross

MT-90 is not recommended by Redline for use in your transmission with its hypoid pinion gear. https://www.redlineoil.com/product.aspx?pid=46&pcid=7

Whine typically indicates worn bearings. How many miles on your gearbox?

951pcarguy

Thanks for the info, so what type of Redline (or other) gear oil is recommended? Past posts here on Rennlist state both GL-4 or GL-5 are OK, so maybe these posts have been superceded by more recent analysis?

I've put 40K on AOR transmission that I bought used in 2011, with ~50K before that.

rlets

Yikes! I have 3 years of track use of my 944 / LS1 / AOR car with MT-90. No issues. But Kevin is correct that Redline says it's not recommended. Looks like they say 75w/90 NS is for hypoid gears but doesn't necessarily work well with an LSD. Perhaps the Kendall GL-5 or Mobil1 LS oils are a better choice...

Rich

KevinGross

There are a lot of good lubricants out there. Especially if your car is street / autox only (versus track), the most important thing you can do is *maintain* the fluid, that is, change it on something like a scheduled basis, and worry less about which manufacturer's product you use. I've used and recommended Mobil 1 LS 75W90 and it works well in all climates, use it in my own car. Many have had very positive experiences with Swepco although I find it makes shifting a transmission very stiff in New England when it's cool outside, until warmed properly.

Look for a name-brand fluid that has the right viscosity numbers, 75W90, and is spec'ed as suitable for hypoid pinion design, which puts much greater stress on the pinion and its lubricant than spiral bevel pinion designs. Synthetic fluids have a number of benefits, most importantly greater viscosity stability across the full range of temperatures. Good luck!

mj951

I've been using Red Line 75W90 in my 89 Turbo for several years now without issue.
Back of the bottle spec: SAE 75W90, API GL-5 + Hypoid Gear Oil.

arthropraxis

+1. I tried swepco, redline MT-90 and RL 75W 90 trying to fix what I thought was a transaxle issue. Changing gears felt the best with the RL 75W 90 and I never noticed and issue with the LS. There is a friction modifier that can be added.

I have used Mobil 75W 90, LS after Kevin Gross rebuilt my transaxle, and it has been fine for the last two years. It is time to change the fluid and will go back to the Red Line.