The Stovebolt Site Forums The Shop Area Driveline Synthetic gear oil for older trans and rear end?

I am thinking of switching to one of the high end synthetics for my dif and tranny in the 1969 C-20. The change seems to have a lot of pluses. The only minus is cost. Anyone have any considerations to relate?

best wishes,

Les

From a wear standpoint, I think it will make your transmission last forever. I use 50W synthetic in everything from my jalopies to my over the road trucks. However, it is very slick and reliable people have reported that it is so slick that their synchronizers don't function like they want them to. For me that presents no problem because I don't use a clutch on anything and don't rely on the synchronizers.

That would be me Ken. And I’m just reporting my results, being in the oil business I’m
sure you’ve seen a whole lot more examples then me.
The syncro components and gears in the SM420 four speed are rather massive. You can
make a quick shift from 2nd to 3rd and everything seems to keep up ok. But, the shift
from 3rd to 4th needs to be a little slower or you will grind the gears. This is simply
because you‘re trying to match up the speed of two components that are quite heavy. The
shear strength of the oil is what drives the syncro and if it‘s to slippery then there isn‘t
enough friction for the syncor ring to grab the drum and pull it up to speed. This is true
using a heavy mineral oil such as 85w-140 but is much more pronounced with the
synthetic oil. I think the more massive gears need a higher shear friction than the
synthetic oils have.
With the smaller less massive syncro drum and gears in the 3speed transmissions and the
newer four speeds such as the T-5 you might not have a problem using the synthetic oils,
I don‘t have one so I can‘t say for sure. However I’ve read articles reporting similar
synconization problems in sports car transmissions using synthetics.
I'm convinced that the synthetics reduce friction better than the straight mineral oils and
do greatly reduce wear. I have friends in the engine business down in Indianapolis and
they do a lot of race engine rebuilds. They told me that the engines with the synthetics
show almost now wear compared to the engines using the older mineral oils.
In the engine or rear axle I can't think of any reason for not using them unless the rear
axle is a limited slip unit that uses a friction device.
Denny Graham
Sandwich, IL

Denny -

I don't know if you're playing with my head, or if it is true ...

Maybe you have something there. Jo Ann and I drove our '54 4-spd around town for about an hour on Christmas day - the first time it's been on the road since I drained the tranny and and filled with Lucas 85-140 gear oil. I noted to myself as I was driving that I could feel the gears more than usual as I shifted. I didn't grind any gears. I shifted slowly as I felt the gears go into place.

One of the lube additives I encountered many years ago was a product called "Slip-G". It was basically Molybdenum Disulfide in a lightweight hydrocarbon carrier like Mineral Spirits, and it was marketed as a rear end lube additive. If it sat on a shelf for a few days, the MOs would settle to the bottom of the bottle, and had to be shaken up well before adding it to a rear end. The SCCA sports car team I was working with bought some, and it immediately lowered the rear end temperature dramatically at the end of the races we ran. Somebody got the brillialt idea to add some to a transmission. Wrong move! It turned a slick-shifting 5-speed into a crash box! The moly pretty effectively prevented the synchros from working, and it was almost impossible to get it out of the box once it locked into the pores of the metal.
Jerry

So, engines and non-limited slip rear ends are fine, but NOT limited slip or transmission (for conventional drivers,) is what I understand from the above. The synthetics are just to slippery. I'd have never guessed that! Thanks for the input!

Be safe out there tonight and happy new year!

Les

I'm just relaying my personal experience with the SM420 four speed transmissions Les. I'm sure they make synthetics for the
limeted slip differentials and you may be just fine in the manual transmissions with less massive components in them. bout
the only thing I could say is to try them if you want and see how they perform in your vehicle. I can't see any reason why
they wouldn't add many more miles to your vehicle if they perform ok with the synthetics.

DG