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Ive been trying to find 80w90 gl 4 for my saginaw 4 speed but i cannot find it anywhere, but i have been able to find 85w90 gl 4. Is there much of a difference between these two. I live in california so it is never really that cold, and my chevelle has no heater so i dont ever drive it when it is cold. Any thoughts on what i can use? Ive used the search and even google search and could not come up with a definitive answer.
 

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Just buy the 85W90. It's just gear oil, not rocket science. ;)

Just stay awy from the synthetics.
What's wrong with using synthetics?

I use Redline MT-90 in all my manuals (it's also GL4)
 

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We ran Royal Purple in the '70's M22 when it was rebuilt and it shifted like crap. Especially down shifting. It got drained, and refilled with regular Valvoline 80W90 and it shifted just fine after.
 

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I think either weight would work just fine for you.
 

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What's wrong with using synthetics?

I use Redline MT-90 in all my manuals (it's also GL4)
===================================

The issue used to be that older gen syn were too slick proper upsetting operation
of syncros in the older gen muncie trans.

But fast forword to todays newer gen syn oils and they fixed that 1 of 2 issues wiehn runnikng syn oil which was to change the coeffcient of friction within the syn lube
to work specifically with older gen manual trans likel Muinciew.

The other /2nd issue though not a big problem in most cases is the fact that on a molleculcar lvl the collecules of syn oil are smaller then convcentional oil has making it more suseptable to leaking in an original never restored 35-40yr old Muncie trans.

I also tried Redlines MT90 syn GL4 trans lube in an original never restored 43 yr old Muncie from a 68 chevelle ss 396 and it worked fine but did weep a bit thru it's original gaskets on a hot day .

No real drips,just enough weeping of the syn lube to collect some dust/dirt .

On the trans lube in question ,there's not hardly any dif between 80w-90 vs 85w-90 both being GL4 so the 85w-90 is fine to use.

There's also Sta-Lube 80w-90 GL that's avail by sp order from Napa ,you can also find it from mult sources online too.

Scott
 
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