As far as mixing them, you should call Redline and ask what they think. This is from their site....

Should I flush my engine or driveline before using Red Line?s synthetic lubricants?

A flush isn't necessary or advised, as Red Line?s products are compatible and will mix with other synthetics and petroleum products.

So they would be compatible. I have heavy shockproof in my 93 turbo. It works good. It is sluggish until it gets warm but it works great. Maybe go with the light shockproof.

 

i'm running a mix of the lightweight shockproof and the mt90. it's only been about 1000 miles and the transmission was in pretty good condition to begin with. other than the fact that nothing is broken yet, i cannot say it has made a difference (switched to a c56).

i used to mix it on my wrx (lw shockproof/pennzoil syncromesh/castrol hypoid C) and it was pretty nice, so i figured i'd try it with the MR2.

 

Does it make the shifter noticeable smoother once the oil warms up a bit? It's going to be 110 every day real soon so I'm not too concerned about temperatures.

 

again, can't say if it made any difference because i just switched to this transmission, but for the WRX it made everything noticeably smoother.

 

I just put in 1 quart lightweight shockproof oil and 1 1/2 quarts of MT90, The lightweight shockproof seemed a wee bit thicker than the MT90, but not nearly as thick as the Heavy Duty shockproof is. I'll put up a drive report soon, I might just go for a spin now actually...

 

The shifting seems just a little bit smoother. Not a night and day difference, I probably wouldn't have noticed if I hadnt driven the car already today. There is no noise going into reverse though, and the shifting did seem to get smoother the more I drove the car.