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I keep seeing people on here recommending synthetic gear oil for old manual transmissions that were designed for 80w90, but is that really a good idea?
My '89 Toyota pickup with 240k miles started grinding 3rd gear after I switched to synthetic GL-4 like everyone suggested, and when I drained it after 2k miles the old brass syncros looked more worn than any dino oil change I've done over 15 years, so should we be sticking with the old stuff for high-mileage boxes or are these just isolated failures?
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wright.leo1d ago
You're blaming the synthetic gear oil, but that grind was probably already on its way with 240k miles on those old syncros, not caused by the fluid itself.
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sammartinez1d agoTop Commenter
Holy crap, 240k miles? That's insane, I didn't even catch that detail. @wright.leo you might be right that it was a ticking time bomb at that mileage, but I've seen people run those old trannies way longer with synthetic fluid and no issues. It's funny how everyone swears by synthetic gear oil for cold weather shifts but then acts surprised when it exposes a weak syncro that's been held together by thicker fluid for years. I think the real issue is people don't flush their old fluid out before switching, so all that crud gets stirred up and settles somewhere it shouldn't.
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rileyprice16h ago
So you really think the fluid had nothing to do with it, even after 240k miles of running on dino juice? I mean, yeah the syncros were probably worn down, but that thick old stuff was basically a bandaid holding everything together, right? When you swap to a thinner synthetic that flows better in cold weather, it's gonna slip past those worn parts easier and let whatever grinding was gonna happen, happen. Have you ever actually pulled apart a high-mileage trans after a fluid swap and seen the gunk that settles in the bottom? I helped a buddy do that last year and it was nasty, chunks of stuff that had been floating around for years. Not saying the fluid caused the damage, but it sure didn't help the situation by letting all that crud circulate freely.
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