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Originally Posted by dknv
what's the reasoning behind that formula?
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I tried straight 75w90ns in the tranny and 75w90 in the rear diff. The 75w90ns doesn't have enought friction for the synchros, so while it's great for the bearings in the tranny, the synchros tend to slip too much, requiring very gently operation of the shifter to keep it from grinding. (I can only assume this is what Evan's refering too since he didn't mention what "problems" RedLine causes.)
The shockproof has a different friction modifier, and it's designed for racing transmissions. It has this crazy high surface tension (that stuff won't even drain into my oil catch pan when I last did my fluid swap, I had to push it inot the drain with a towel!!) that prevents the oil from foaming in the transmission, and leaves a nice heavy protective coating on the gears. After trying the 75w90ns and hating it, I tried pure Lightweight Shockproof. The difference was night and day! The synchros grab reliably, and a lot of the notchynes went away, making it much easier to shift. However after reading that the Lightweight shockproof isn't necessarily the best on the bearings, I decided to try what a lot of other people have done, and homebrew up a RedLine cocktail.
Lots of people say to use 3/4 RedLine 75w90ns and 1/4 Heavyweight ShockProof. However, Summit was out of Heavyweight when I was there last, and since I really like the pure Lightweight, I figured I'd try the 3/4 Lightweight ShockProof 1/4 75w90ns mixture. Bascially I want to lighten up the oil a bit for bearing life, but I don't want to lose the great feeling synchros/protection the pure Shockproof gave me. Next time, I may try a 50/50 mixture... or if Summit has the Heavyweight, I'll try the other mix.
As far as tranny protection, when I recently swapped the fluid at 30k miles, there was less "sludge" on the magnet from the last 25k miles than there was when I originally swapped fluids at 5k miles. So I figure that stuff's gotta be doing a pretty good job.
FYI: RedLine makes an "UltraLightweight ShockProof" gear oil... that may be a good non-mixed solution, but I've yet to see it at Summit.