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 G
Sandwich, IL