Quoting DynastyLCD:
wait, wait, wait.... i've got a question here. if GL-5 destroys synchros, why are people using Diaqueen? isn't that GL-5?
personally, my transmission comes out and goes back in so many damn times that i just run cheap sh*t... typically whatever 80w90 gear oil i can get at the cheapest cost. i've got the Kia version of Diaqueen i was planning on trying, maybe i'll hold off considering the GL-5 thing.
i used to use Shockproof Lightweight, and after tearing apart enough of my own transmissions, i found that what others said about it was true - it can clog the smaller oil passages. my recipe is: tcase and rear diff get heavyweight shockproof, transmission gets 80w90 or MT90.
i've never liked Synchromesh. i feel as if it's too light and too thin. makes for some excellent shifting, but i could see how it wouldn't protect the way MT90 or another quality gear oil would. there's even some schmuck around here who tells people to put 5w30 in their DSM transmissions... WTF!? i've been told in the past by a VERY knowledgable DSMer to either pick Synchromesh and beat up the transmission or use gear oil and deal with the slower shifting. pick your poison, i suppose.
if/when i ever actually construct a real built transmission with Evo gears or dog cuts, i'll go MT90 only in the transmission. i agree whole-heartedly with that idea, just never had a transmission worth it. the Synchromesh i never liked. regular gear oil has treated me well but i don't typically put anything more than 5k on my car without the transmission coming out for something or other, so it's difficult to base longevity from that.
Yes, PLEASE DON'T put engine oil in your trans. Working at Dana I've played around with plenty of trans and diff oils and I would never put 5W30 in my transaxle!
Yes synchromesh is thin but for DD (again Daily Driving) it's fine. I've tried MT90 and it's just too notchy for me (maybe my sychros are worse than I thought).
You are correct though, unless your racing, it's mostly preference as to what you want to run. Some oils have "friction modifiers" and what have you but it's all preference, you'll get 10 different people telling you that they all run different fluids.
You can get synchromesh that IS GL-5 and you can get synchromesh that is NOT GL-5... read below.
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I'm not saying synchromesh is the best thing around because I'm sure it's not. Just throwing out some knowledge and experience I've had with it.
Zach.