Weird.. It's summer here in TX and I'm running 10w30 in the AX15. It shifts pretty good.

 

If he likes how 15w40 shifts now, MTL is gonna be super notchy!

I agree with the assessment of using gear oil in a gear box though. Pennzoil Synchromesh is a cheaper option

 

15w40 feels like BUTTER in the thing. It really feels a touch better than I remember MT-90 feeling when it was new.

It was like $12 for the fluid change which is sweet, but I probably should change it every 5K miles with the oil since it's conventional (I have a nice fluid pump, it literally took me 10 minutes to drain & fill including waiting for it to drain).

My trans is pretty worn though, lots of miles, and it was definately a trail rig before I bought it so who knows how it was treated. I am pretty sure the first fluid change was around 160k miles also.

If your trans was well maintained I have no doubt that 10w30 could feel just as smooth

 

Random Update;

15w40 is too cold for sub 20*F mornings. Takes about 5-10 mins before the trans wants to shift normally. Shifts great after that, or when it's warm out though.

I am going to probably swap back to MT-90 because of it. Going to ride it out till the next oil change (2-3k miles) and see if I can detect a change with the MT-90 when it's hot out

 

I've been really happy with MT-90 when it's hot out. I've wheeled in 100*F days and it's smooth as silk.

 

This is good to know. I should really get around to changing mine, as it is still sitting in the bottle, and I am running 10w30 in there right now. Still works fine, and better then the old gear oil that was in there, but I didn't spend a bunch of money on the MT-90 just to let it sit in the bottle.

 

I agree with Salad on this one. Been using Pennzoil Synchromesh for years in many different vehicles. It is what is in the Jeep right now. Never had any complaints. The only manual trans I dont use it in is in the Bug. It gets ATF instead for other reasons.

 

Its funny you say that, GM manual transmissions take ATF as well. My T/A takes Mobil 1 Syn ATF

 

I wouldn't pay too much attention to that. The idiots shoved some expensive Allison ATF into the ZF6 transmissions in 2500 and 3500 trucks and didn't specify a change interval.

 

Great info on here guys, thanks. I put MT-85 in mine last winter (nudged by something I read somewhere I can't remember), was a noticeable improvement over what I assume was GL5 from the previous owner. I'm in Missouri where we can see all four seasons in the same week, does anyone think MT-85 was a bad decision over MT-90?

 

As long as it shifts well and doesn't make noise, keep at it