not sure on a custom tranny,
i assume all the gear ratios and box is self is basically a stock one with a 1.5 lsd in it. (saying that the gear set has NOT been replaced at all)
flywheel and clutch have nth to do with it so ignore those.
motul gear 300 is much thicker than regular honda manual fluid.. so... its probable to damage the transmission (over continuous driving/ time- result is the gears eating away each other, metal filing, burnt fluid, sludged fluid) but since you have a LSD in there, im not that 100% sure now.
75w90 is used across the board on most cars with standard boxes today, cept honda. honda has their own blend of fluid which is perhaps quite similar to regular 10w30 gear oil (cept synthetic) (fact: all hondas usually before mid 90's use regular 10w30 motor oil as standard transmission fluid, DC2 being no exception. Synthetic manual fluid for the newer cars)
so yeah... i mean i've been dicking around with fluids for quite a while, not to mention i was working for a honda specialist for a while last year.. (got bitched at for putting in 75w90 elf synthetic fluid)
with the current set up, its quite similar to the current civic Si... (which has a LSD) so i'll look into it tmmrw, and let you know. but i could really bet on it, that the car's tranny is best (and would be kept in better shape/ last longer) if you used the oem honda manual fluid.
just my three cents ``