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Manual transmission fluid?

7

78 GMC Jimmy

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Anyone know what the lightest fluid that can be used in a 78 K5 manual trans? I switched from something heavy to something lighter and it helped a lot. How light can I go?
 
I had 80/90 and it was hard to shift into a gear. I put an additive in that helped a little (AT-205). Ran that for a bit. Drained the fluid and put in 3 quarts of 75W-90 Synthetic gear oil. I thought that would be enough, but it was not, so I topped it off with some of the 89/90 that I had just drained out. If I had to guess, I added 1 quart of the 80/90. I would like to improve the shifting more.
 
78 GMC Jimmy said:
I had 80/90 and it was hard to shift into a gear. I put an additive in that helped a little (AT-205). Ran that for a bit. Drained the fluid and put in 3 quarts of 75W-90 Synthetic gear oil. I thought that would be enough, but it was not, so I topped it off with some of the 89/90 that I had just drained out. If I had to guess, I added 1 quart of the 80/90. I would like to improve the shifting more.
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My transmission had too much wear for the mileage and had to much play.
You can give it a try and see if it gets you what you want.
That transmission was never known for it's smooth shifting by the way
 
I have been running synthetic Dex6 in my sm465 for about 6 years without issues

If I was towing I may consider switching, but that oil is an equivalent spec to what the 4500 uses
 
Bent77 said:
I have been running synthetic Dex6 in my sm465 for about 6 years without issues

If I was towing I may consider switching, but that oil is an equivalent spec to what the 4500 uses
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Does Dex6 improve the shifting for you, VS 75W90?

How much does it take?
 
I think the only place you'll notice shift difference is when it's cold.

Mine seems to warm up enough in the first quarter mile or so (around freezing ambient) for shifting to get to "normal".

The difference between cold and barely warm is so stark, I don't see how lighter fluid would make an appreciable difference in how hard it is to get into gear, compared to after the spec'd fluid is warm. When cold, it simply won't go into gear without significant force. After that though, it takes little effort to shift.

Never tried anything else, so I'm open to learning, but if it was an amazing difference, I think it would be a lot more common.
 
I have only ever run 80w-90 GL5 in my SM465, and have no reason to switch. I'd rather have the thicker oil for more of a shock load cushion, if that makes sense. I do run the higher end Lucas stuff though.
 
dyeager535 said:
I think the only place you'll notice shift difference is when it's cold.

Mine seems to warm up enough in the first quarter mile or so (around freezing ambient) for shifting to get to "normal".

The difference between cold and barely warm is so stark, I don't see how lighter fluid would make an appreciable difference in how hard it is to get into gear, compared to after the spec'd fluid is warm. When cold, it simply won't go into gear without significant force. After that though, it takes little effort to shift.

Never tried anything else, so I'm open to learning, but if it was an amazing difference, I think it would be a lot more common.
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The only advantage you will see is the synthetic fluid whatever you pick, because it doesn't thicken the same when cold.
That is why I try to use synthetic for everything
 
A lot has changed since 1974. I’m sure the reason they went to Synthetic and lighter fluid in the newer transmissions are valid
 
I thought we were talking about a 78 sm465.

A 60's vintage np435 used straight 50w.
 
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