Differential Oil

[SUX2BU99]

Getting the rear diff oil changed and a new cover put on tomorrow. I bought RedLine MT-90 gear oil. It's a 75W90 synthetic and it says it's good for GL-1 to GL-4 designation and replaces 75, 80 and 90 weight oils. It says it's for manual trannies and transaxles and says it's NOT for use in rear diff's with a GL-5 designation. Counter dude phoned his buddy at a shop who does rear ends and says that's only a concern if my truck has posi, which it does not. It's a truck 12-bolt. His concern wasn't of the non-GL-5 designation (whatever that means) but moreso about the synthetic not playing nicely with clutch-type limited slip rear ends.

I'm sure my 12 bolt was made long before the current GL-5 classification. Do I need to be concerned at all?

[Dragon]

Again you should be fine when it comes to the GL-5 rating(Unless the gears are a recent addition from the last few years or so)....

His main concern is warranted, as the thinner make up of a Synthetic can cause problems with the Clutches(Allows more friction), which can cause them to prematurely fail....  Same can Also be said for a Posi-Setup, but I have found Red-line High Performance to be alot Quieter, than other Gear Oil I have experienced in the past....

To be on the safe side, unless you are expecting to go with a new Rear Setup soon(next few months, to a few years, if not longer/shorter), I would suggest you stick with a Traditional Non-Synthetic Gear Oil....

[SUX2BU99]

I was reading on Red Lines website and they do make a specific 75W90 gear oil, but the store I was at didn't have it in stock. Maybe I should just stick with regular gear oil. Castrol makes their Hypoy C stuff for about 1/2 the cost of the Red Line. $5/qt. vs. $9. But those were a bargain compared to the Quaker synthetic which was $14/qt!

[VileZambonie]

Hypoid gears have a sliding spiral cut contact pattern and the pinion is below the center line of the ring gear. You should run an 80w90 or if you want to run synthetic 75w140. If it's a limited slip application add the friction modifier.

[SUX2BU99]

For sure. I went out at lunch and bought some of the regular Castrol stuff, 80W90 and will return the Red Line. I don't want to use the wrong product for the application.