I have 5 quarts of redline D4 ATF synthetic (rated at gl-4) left over from my 4eat 04 wrx. When checking my sti tranny fluid level, it is halfway between L and H. Im thinking this might be the cause of some subtle grinding here and there and some 1-2 shift problems at redline. For example when im at the strip, the 1-2 shift takes wayyy too long and it is as if I hit a barrier between the 1-2 shift before the synchros work and it goes in, still with some difficulty. When the car is cold I have to double clutch just to downshift into 2nd at a low speed. This is in 90 degree weather, god knows how itll be once it starts getting cold.
So my question is, could I add some gl4 redline synthetic atf to the stock oil to fill it up? Other gl4 rated tranny fluids such as redline shockproof and gm synchromesh are used in the tranny without problems. I would only add 0.5 qts at the most. What do you think?
PS-I know hypoid gears in the front diff require gl5, just trying to get the synchros to work better.