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Thin-est oil for a manual trans?

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78 GMC Jimmy

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1978 K5 Blazer. Manual trans...

Whats the absolute thinnest oil that can be used, to make shifting easier?


Thanks.
 
Couple things:

Are you using synthetic gear oil?

You could use 50W motor oil or even 30W in a really cold climate I suppose although I don't think I would. In my old 4 speeds I use 85W90 GL4 gear oil and they all shift fine.
 
Chickntrk said:
Couple things:

Are you using synthetic gear oil?

You could use 50W motor oil or even 30W in a really cold climate I suppose although I don't think I would. In my old 4 speeds I use 85W90 GL4 gear oil and they all shift fine.
Click to expand...
It did some extra research into this while back. The syncros seem to be the biggest concern on these transmissions so sticking with something that was intended to be in the first place seems to be the best option.
 
When the trans is warm, it shifts better. I switched to about 50% 80W90 gear oil and 50%:

Ultra1Plus SAE 75W-90 Synthetic Gear Oil, API GL-4 (copy paste)​

The transmission still doesn't shift the way I would like. You have to match the speeds pretty well or it wont go into the gear you want. Like especially when its cold, I have to practically come to a stop and rev the motor up to get it into 1st (2nd) gear. Its almost as the though the clutch doesn't fully disengage and your trying to shift without using the clutch. However - the clutch does fully disengage, I tested it. So the thinner warmer fluid seems to help a lot. Are my syncros worn? When I changed the fluid last, the fluid looked new, not full of metal or dirty or anything.

Thoughts?
 
If it makes any difference, it does shift easier at higher speeds in higher gears. Still not great, but better than at low speeds.
 
The clutch disc could be bent, not allowing the input shaft to slow down properly.
50w gl3 or 4 is good, hard to find.
 
when I pulled my last 465 apart, the weld on the shifter was broke. Probably from powershifting it
 
Even with new syncros this type transmission will not shift as smooth as most modern types.
A SM/CH-465 is a design that was meant for medium-duty on down truck platforms back when most trucks truly worked for a living.

The Blazers and half-ton trucks just shared what GM offered rather then go through the expense of designing another transmission because the passenger car type 4-spds would’nt cut it for truck use and were not geared properly.
GM did offer the NP-440 in the mid-eighties but that was for the benefit of the O/D.

The 465 is a truck transmission and will not ever shift as smooth as most car type transmissions mostly due to the gear ratio splits.
It’s hard to match the shaft speeds going from third gear at 1.68:1 down to second gear at 3.58:1 - that’s why it’s hard ( assuming the trans is healthy ) to downshift into 2nd gear at speed - don’t force it because it was never designed to operate like that.
 
Yes it's synchronized...... It might like Mobil 1 Synthetic gear oil....

Might be time for new Synchro's

It's an old designed Truck Transmission with long throws ....

It never will be like driving a BMW or Porsche....
 
Pull the shifter out and see how loose the end is. The roll pins will wear and the whole unit can be tightened up with a small weld. It should help make the shifting a bit more precise but it's still a dump truck trans.
 
The 465 can be run with 50wt gear oil. I did it a few years ago over a winter and it did improve the cold weather shifts. The Amazon link above is good stuff.
 
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