I was at http://www.finishlineperformance.com and planned on getting tranny oil...my question is, they have two. MT-90 and MTL...which is better and why?

Call them and ask or go to the Redline web site and read about each type. One thing to be careful of if you're also going to change the differential oil is to get the type of oil for LSDs (assuming your model came with an LSD).

I have an '05 GT 6-speed. From what I see, most people with 6-speeds are getting MT-90...

The MT-90 is a higher weight and is the one you need (I use it in my 8). I ran (and have 2 bottles still laying around) MTL in my miata. The MTL is a 70-85 GL4.

Thank you very much max5. :)

The MT-90 matches the recommended viscocity. I have read reports where people swear by a *mixture* of MTL and MT-90 as having the ideal shifting properties. I did read someone on this forum not long ago who claimed a 2-to-1 mixture (2 parts MT90 to 1 part MTL) was just right. I'm using just MT-90 and it seems fine. It might have something to do with climate too. I imagine adding the lighter MTL might make more of a difference in a cold climate.

How ofter should you change your transmission oil regularly? Besides upgrading to something like Redline and Purple, is there a regular period for tranny oil change?

30k miles, in you owners manual. also rear diff.

beers

I guess you can't go wrong with the Mfr. service interval. But you will notice on the first change, whether it's at 5K, 10K or 30K, the fluids will be almost silvery from the amount of wear metal from break-in. There's no oil filters, just magnetic plugs and they obviously only grab particles over a certain size. I don't know if all this metal makes any difference but I like the idea of getting it out of there before 30K, at least on the first change.