[DeleriousZ]

i had that exact same problem with my previous transmission only it was into 2nd not 4th.... does it happen when downshifting from 5th to 4th as well? if it does, i'm afraid it's most likely the syncro's and you're going to need to find yourself a new tranny or someone who's good at rebuilding them.

as for oil, probably synthetic would be your best bet, that is, if the previous owner was using it as well.. i've heard that car's don't like them being switched back and fourth all the time.. that could be total bs tho i'm not 100% sure.

also, you might want to check out if the timing belt/water pump has been done before.. you're supposed to change it every 60k so if this one hasn't had it done, you might want to look into doing that.

if the car has been well maintained the turbo's should still be running strong, maybe check to see what you're boosting, i'd highly recommend installing an aftermarket boost guage when you're boosting anything over stock.. if you're not boosting up to what you think you are, that could be the trouble with the 'lack' of accelleration..

anyway.. welcome to the forums, hope you find what you're looking for