The Stovebolt Site Forums The Shop Area Driveline Synthetic gear oil, Slippery or just to Slippery?

Well got almost 1,500 miles on my new rebuild, second oil change and she’s running
strong and taint used enough oil to even mention. Real happy with the 216, she pulls
strong and runs nice and smooth.

Even though it was nice and tight and not making any unusual noises and really didn’t
show any signs of wear I chose to rebuild the SM420 at the same time. Seemed like a nice
idea to start out with a completely new drive line from the fan blade in the front to the
differential cover in the rear. I used a Novak‘s parts kit for the rebuild, and the gear box
was in excellent condition, absolutely no sign of wear anywhere. Thinking I wanted to go
first class with everything I opted to refill with Mobil Delvac Synthetic Manual
Transmission fluid 50 for extended drain use. This was recommended as it was being
used in a fleet of over the road trucks with manual transmissions

After driving her for a couple of months now I have one thing that’s really buggin’ me.
Shifting from 3rd to 4th before the rebuild was silent, never had a problem with the syncro
catching up. When I brought her home 8,000 miles ago I flushed and refilled with Citgo,
Mystik JT-7 85w-140 gear oil which I’d used in synchronized manual transmission for
decades before. Now with the Delvac Synthetic oil the syncro is having a hard time
keeping up with the gear change. It’s fine if I shift real sloooooooow or double clutch but
a normal shift almost always get’s a tooth or two.
I
’m thinking more and more that the synthetic with it’s increased lubricity is the cause of
the syncro ring not grabbing the cone like it did before.
Driving season is about over for me and Bluebelle. I’m putting her down as soon as the
snow and salt starts to fly. So I’ll be draining the Delvac synthetic and refilling with my
old standard JT-7 for the 2012 season.
Any thoughts on this or better yet any personal experience?

Denny Graham
Sandwich, IL

Synthetic has been showing issues in all sorts of ways. Before putting it in an AT, you need to make internal changes. The lack of seal conditioners in synthetic oils can cause shrinkage (!) and then leaks where there where none before. For a manual tranny, the synthetics just don't have the shear factors of a mineral oil. It's definitely NOT your imagination!

I agree that synthetic is slippery and could possibly cause trouble with synchronizers doing their job. I haven't had much experience with synchros because I never use a clutch except for untracking. I even shift 3 on the column without a clutch. But I do know that transmissions shift much easier with 50W synthetic than mineral or GL5. We have logged millions of miles with our Eaton and a few Meritor transmissions with this oil. http://www.finalube.com/Product_Data_Files/Fina_Branded_PDF_Files/Synpro_Transmission_50_pdf.pdf And we actually have less trouble with leaks than we did with mineral oil. Leaks are caused by vibrations and the Synpro eliminates vibrations. I was a distributor of Fina oils for 45 years until the lubricants division was sold to Total. However I still have enough of the Synpro on hand to last me quite a few years.

Thanks for that bit of info and the link - very interesting reading.
I would still stick to my findings for passenger cars and trucks - it's easy to find this info on the net and easy to see in practice (as did Don). As a distributor, you would know best that oils for the heavy truck industry use much more robust additives that increased shear rating stocks, as well as seal conditioning additives that are obviously superior than where the first gen oils (as stated in your link). I'm pretty sure newer passenger car&truck rated synthetics have been improved in these areas as well, but there's still a lot of facts out there that support the alternate.

After studing this issue for a few years now, I think the best uses for the new synthetics are in newer engines with tight tolerances and in cold-weather application. The reduced viscocity at low temps is a real benefit in the Northern climates.
Right or wrong, but based on the available evidence, I use mineral oils in all my 2000 and older vehicles and synthetics in anything newer.

Ed I’ve heard the same rumor many times about synthetic oils causing leaks and ruining
seals but according to the oil companies who have done extensive research in that area
it’s unfounded. That it would cause problems like that doesn’t make sense to me either
since the synthetics base are the same old mineral oils that we’ve been using all along.
And the rubber and application geometry of the seals and sealing surface is the same no
matter what the application. Many if not most of the new vehicles on the road today are
using synthetics throughout and don’t seem to have recurring problems with leaks.

Well Ken I was counting on your experience with your fleet and your past history in the
oil business when I picked the Delvac in September. The synthetic oil still might not be
the problem with the syncros but it's the only factor that really changed and it does seem
to be a logical approach.
I’ve read several reports of syncro slippage problems with Triumph overdrive
transmissions after switching to synthetic oils and the problems go away after switching
back.

The only way to really tell is like I said, I'll drain her out this winter and refill for next
spring with the JT-7. Should know right away. And like I said the cone didn't show any
wear at all and the machine tooling marks in the original syncro ring were still crisp and
well defined, very little wear.
Another possibility is that the difference in the viscosity of the 50w synthetic vs the 85w-
140 multi-grade. The heavier oil might pick up the syncro faster than the lighter oil. That
along with the difference in lubricity could be what's causing the lag.

We'll know next spring. Winter is time for those basement projects that I put off when the
weather is nice above ground.

Denny Graham
Sandwich, IL

I'll bet the viscosity change is/was the cause...

cm

Like Denny, I thought I'd do the best for my former ride (46 Dodge) and use synthetic in the tranny. It had always shifted fine, but with the synthetic I noticed that after coming to a full stop I could not immediately downshift from 2nd to first or else it clashed. It had never done that before. If I waited 3-5 secs it was fine. I guessed at the time that the gears were spinning easier with the synthetic and tool longer to come to rest.

I switched from the 80 weight gear oil to the 85W140 in my rebuilt four speed and it definitely made the synchros work faster. Also, the gear oil says it is also for limited slip differentials.

Okey Dokey, gonna write the last chapter of this novel..

I drained out the Mobil Delvac Synthetic Manual Transmission fluid 50 and refilled with
Citgo, Mystik JT-7 85w-140 gear oil, which I've run in all of my rear ends and manual
transmissions for many decades now, and guess what................
NO MO GRINDING THE GEARS!!!!!

Old Bluebelle shifts like a new transmission again. I’ve driven it a few hundred miles
mostly around town since the switch just to prove out the difference and it shifts as
smooth as silk syncros working just as they should

So from my personal experiance with a brand new SM420 rebuld, stay away from the
synthetics in the gearbox and stick with the old tried and true multigrade gear oils.

Denny Graham
Sandwich, IL