VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 140w Oil in Various Components?

Hello everyone,

As usual, I need to ask for some assistance from those with more experience than me.

I've been reading the forums regularly, and with my 1941 Chevy up on the hoist for some repair work, upgrades, and maintenance, I have been tackling the issue of what oil to use in the transmission, U-Joint, torque tube, and rear end.

I have found other threads recommending a straight 140w oil (GL5) and after an exhaustive search (it's nearly impossible to find in Canada), I was able to source a 5-gallon pail of GL5 140w. This is straight 140w, not the 75w140 that is a bit easier to find. My understanding is that 75w140 will run pretty thin under different temperatures versus straight 140w.

My plan is to use this in the transmission, as I notice mine, while shifting up just fine, has a slight tendency to grind a bit on the way back down, especially when I shift into 2nd at higher speeds. I have just become used to double-clutching it and that makes the shifting perfect, and while I will always shift it that way from now on, is it still a good idea to use 140w?

Same question for the U-Joint? A technician I hired last year changed the U-joint, but having no new seals he used the old ones, and most of the 80w90 oil he put in there leaked out. I now have new seals for the whole torque tube, so shall I take the opportunity to fill the U-joint with 140w as well?

Is this also okay to use in the rest of the torque tube? What about the rear end?



Finally, a bonus question: My front shocks leak badly. I'll eventually have them rebuilt, but they drop out any oil that is put in them within minutes. Any issue with using a heavier grease in those seeking better performance from the shocks without leaking? Fortunately my rear shocks do hold the oil very well (and I think it is 80w90 in those right now).


As always, thank you for your help.

After 1937 Chevrolet did not specify #140 . #90 was specified. 140 is very "thick' when cold.

A 75W140 and an SAE140 are both the same thickness when hot. They are both 140 when HOT. Straight 140 does not change anything except it will be crazy thick when cold and likely cause some shifting issues. If you want an SAE140, stick to the Multigrade 75W140.

Viscosity ratings confuse many people. SAE 140 is rated at 140 at operating temperature which is around 200F. This is the engineering spec it must conform to. When your car is say 32F(0c), it could be as thick as SAE200 and will be this thick until it warms up to 200F which will take hours of driving. 75W140 at the same 32F (0c) will still be thicker than 140 but since it has the COLD CHARACTERISTIC of a 75, it won't be as thick as the 200 that the SAE140 hit, but might only be around an SAE150.
Both the SAE140 and the 75W140 are thicker than 140 when cold - this is what confuses many people. You gain no advantage using SAE140 as even the 75W140 is always at LEAST as thick as 140 and never gets any lower than 140. It just does not thicken as significantly as SAE140 when cold.

SAE140 Thickens to SAE 200 when cold and thins down to SAE140 at 200F
75W140 Thickens to SAE 150 when cold and thins down to SAE140 at 200F

The cold temps I provided are an example to show the principle of the viscosity change as they will also vary by manufacturer and brand. All the oil must meet the engineering specifications which are a range. Some brands are on the high end of the range and some brands on the lower end of the range depending on their chemical concoction, so you will see some variation of performance as some will "feel" better in one transmission than another.

Suggest a separate post for each topic like your shocks.

There is one other factor that should be understood when comparing multi-grade to single grade lubricants. That is VI (viscosity index) improvers. They are added to base stock oil to change the viscosity/temperature characteristics. Simply they are polymeric compounds that are soluble in oil and change shape with temperature. So when cold they don't change the viscosity of the oil much. But as they warm they become more difficult to move past each other which inhibits the flow of the lubricant mixture and raises the viscosity compared with oils without them. [Think of them as being in like a ball when cold and waving their arms when warm. How fast can a group of people pass other people when they have their arms as their sides compared to flailing them.]

That is why as Tim wrote:
SAE140 Thickens to SAE 200 when cold and thins down to SAE140 at 200F
75W140 Thickens to SAE 150 when cold and thins down to SAE140 at 200F

Because they are fairly large molecules VI improvers are more susceptible to shear than the base oil. That can be envisioned as cutting off part of their arms. So generally multi-grade oils are more likely to have the viscosity reduced during use than single grade lubricants. In high shear conditions the multi-grade lubricants will have their working viscosity reduced faster than the single grade lubricants. Transmissions and rear ends can have higher shear than engines.

Another important factor is that transmission and rear end lubricants are not changed nearly as often as engine lubricants. But that is another topic for discussion.

I definitely appreciate the feedback and I hope you won't mind if I play Devil's Advocate here a bit...

In this post: 41 sd grind from 1st to 2nd only when hot it was suggested that this person use straight 140w in their transmission, but it would seem that I'm being warned away from it? I'm not upset by that or anything, just want to understand what might cause the different advice.

Also, it seems that in that thread it is referenced that Gene recommends synthetic 85w140? I am not sure if I can obtain that here in Canada although I can obtain synthetic 75w140 if that would be better. I guess I should also mention here that I have now noted that some folks have concerns with a GL5 oil in a transmission due to extra wear on the brass/yellow metal parts? If true, that makes the right oil ever harder to find here in Canada.

So, just to confirm, you're warning me away from straight 140w and into something a bit thinner? I do have access to 80w90 gear oil, but using that it seems the transmission grinds shifting from 3rd to 2nd, which is how I found that above post in the first place.

I would imagine that none of the transmission, U-Joint, or rear end get particularly hot during operation? Therefore perhaps something like the 75w140 is better?

EDIT: I can find "Royal Purple 85w140" which does say that it is safe for brass/yellow metal and also synchro safe. Maybe that is a better choice, especially for colder starts? I am in Southern Ontario, and this is a Summer-only car, so I imagine the coldest starts would be at about 5C/41F. They also make it in 75w90, 75w140, SAE90 if any of those are better choices for a worn transmission, U-Joint, and rear end.

The bit about GL5 and GL4 and brass is a bit outdated, but still gets circulated a lot. When the cars were new, there weren't many kinds of gear oil, GL5 didn't exist, and it made sense to put the same thing in the transmission as the rear axle. Today it doesn't make any sense at all.

Today there are oils made for synchromesh. It is because the regular gear oils for rear axles got too good and too slippery. Slippery is bad for synchromesh. The synchros are so huge in these old Chevy transmissions that some report it working fine with regular modern gear oil. I believe them. But, make no mistake about it. A hypoid gear oil like typical parts store bought GL5 is doing everything in it's power to make your synchros not work. It is just too slippery. Whether it is also eating them slowly because they are brass is a separate issue. The chances are pretty good that it is doing that, too.

In that thread you linked, people were trying to help that guy band-aid a weak synchro, some by suggesting heavier oil to increase frictional drag, and others by suggesting real synchromesh oil to make the synchro's job a little easier. In the end, it turned out the synchro was just completely shot and he had to replace it.

Thank you - This is all coming to a head just before I take delivery of that 140w, so I can return that and save myself the cost/storage, especially if it will run too thick at colder temps and even more so if it eats brass/yellow metal.

Any thoughts on using the Royal Purple 85w-140 for this application? As compared to the 80w-90 that is in the transmission right now and causing grinding, at least. Secondly, of course, would be using this for the U-Joint and possibly the rear end, even though, of course, using the same oil in the rear end as the transmission is not necessarily standard any longer.

I am using Redline synthetic 75w140NS (note the NS) in my Pontiac. Despite what the name implies the oil is sort of thin, so I touch second or third at stoplights to stop the gears before going to first. It is GL5 rated, but intended for use with synchros, and safe for brass. MT-90 is another possibility.

I tend to double clutch too, but can shift from third to second without, no grinding. It could be that your synchro is just shot, like that guy in the linked thread.

Pennzoil, Valvoline, GM, and a few others make synchromesh oil these days. It does tend to be fairly light, usually 75w85 or something like that.

I tried some conventional SAE 140 GL4 from Oreilly, but didn't like it. It was balky cold and took a long time to warm up. Even warm it didn't shift as well as with the Redline.

If I was going to run something more conventional than the Redline, I would get whatever Chev Nut recommends.

Glad you are doing some study and asking questions about this. I agree that the proper gear oil choice can make things work better. However as noted no gear oil will compensate for excessive synchro wear.

I cannot remember how many years ago I learned about the "too slippery for synchros" syndrome. I cannot remember which vehicle it was but I wanted to improve the cold weather shifting. So I put Mobil 1 GL5 Gear lube in the transmission. If you listened when I drove the car you would have thought I was just learning to drive a manual transmission! I don't think it was in the transmission for a week.

Please start another thread and we will discuss the shock oil.

Thank you for this everyone. I'd rather ask too many questions and try to get it right than to wade into the deep end and do irreversible damage. For me, a car of this vintage is living history, so I'd rather look a bit green than risk ruining something that someone else worked to protect well before I ever arrived on the scene.

Given this discussion, I am leaning heavily to the Royal Purple 85w-140 GL4 oil for the transmission and U-Joint housing rather than the 80w90 that was in there or the 140w that I had originally purchased for this purpose. While I think I probably have some wear on my synchros, I don't think it is major wear as the grind is fairly minimal and only on the way down. But, out of a desire to preserve the transmission, if this makes shifting a bit more gentle then I'm all for it. Of course, I will still double-clutch on the way down from 3rd to 2nd as it's habit by now anyways.

Great plan. Let us know how things work for you.