Originally posted by subyluvr2212
Synthetic is simply too slick. There is not enough friction between the synchro cones to allow the gears to engage properly when synthetic oil is used.
I tried Redline 75W90NS for a while, was grinding gears left and right, so upon suggestion, as mentioned before, I used Redline MT-90 (GL4) and 75W90NS, half and half. It pretty much solved the problem
The "NS" doesn't have the friction modifiers and is recommended for racing and offroad use. Could this be the problem? Maybe the regular 75W90 (not NS) is a better option. I just have a problem buying that an oil is too slick. Isn't that what it is supposed to be?
This site lists a bunch of Redline oils. The shockproof ones seem interesting.
EDIT: Well after reading the technical specs on Redline's website, I know better. Regular 75W90 would be used with plate-type LSDs that are IN the oil. 75W90NS would be used on a WRX because both of the LSDs are 'dry'. The front differential is open, so it doesn't need the friction modifiers either.
It also seems that a GL5 oil is recommended by Subaru for the WRX tranny and rear diff. GL5 oils seem to be needed for the slow, high pressure needs of a differential, while GL4 oils work better in a tranny. (That's my understanding)
So you should use GL5 in the rear diff and GL4 in the tranny then right? I don't think so. Remember, the WRX also has a differential in the front tranny. I think the factory recommended GL5 is what's needed all around.
What is really needed is a transmission that seperates the front diff oil from the tranny oil so that different kinds can be used. Oh wait! Doesn't the new 6 speed do that?
I'm guessing that it can use two different oils. It also has a different kind of rear LSD, so it might need friction additives.
After all that, I'm prolly going to use Redline Shockproof LightWeight for my 5 speed. I just put Mobil 1 in the rear diff...it was IN STOCK and ready to rock.