MTF Comparisons?
I agree.
However, 75W-140 GL4 is the same as straight 140 GL4 when it's warmed up.
The viscosity changes as it's warmed up to full operating temperature.
The ''W'' in any oil is for winter, not weight.
And using GL4 oil is the right choice, not GL5 which is too slippery.
So, the oil flows like 75 weight oil when cold and changes to 140 when fully warmed up.
I think that this would be a better choice if it were me, but straight 140 would do the trick too, I'm splitting hairs here...
I'm not a fan of mixing lubes to fix a problem. But if I'm going to try to quiet down an A-833 running Synchromesh by mixing something with it, I'm going to add a straight GL-4 140.
Because:
-You don't know how additive formulations not intended to be combined will react with one another. Very often their effectiveness is reduced. The 140 will have much less, if any viscosity index improver and/or pour point depressants than the 75W-140. Less addictive possibly means a smaller chance of poor additive compatibility. It's still oil, though, so at least the gears will be lubed.
- I don't know of the existence of a GL-4 75W-140. Lots of GL-5s, though. The small amount of EP sulfur/phosphorus additive probably wouldn't make much difference. But, see the previous point.
- Whereas adding a 75W to another 75W will probably do nothing in the attempt to quiet down the trans at cold temps, adding a straight 140 should work on both ends of the temperature scale.
- I think most if not all 75W-140s are synthetics. I don't have a problem with them, but some do. Gear clash due to synthetic is real. And does it play nice with mineral base gear lubes?
If this were my car, and if for whatever reason(s) pulling and fixing the trans isn't happening soon, I'd just run the 140. I ran that in a Super Bee year round in a state where it gets below 0°F in winter. It was hard to shift before warm up. The Mopar guy I bought it from said that was what was in it and I should leave it there. So I did. Not exactly buttery smooth shifting, but it got the job done.