The Stovebolt Site Forums The Shop Area Driveline SM420 Gear Oil - what type to use & fill level

Is Master Pro GL-5 80w-90 gear oil (from O'Reilly Auto Parts) okay to use in a SM420 tranny? Is the fill up to level basically the oil plug on the passenger side up about half way? Thanks.

"An 80W-90, API-GL5 or MT-1 rated fluid is very good. "

Lubricant additions - rear-axles and transmissions

That's what I've been using in all my manual transmissions for 57 years.
Yes the fill hole is the one that you're looking at. It's the
plug in the lower center in this picture:
http://www.pbase.com/dennygraham/image/137528063/large
As noted in the manual that Tim has link to, the level
should be about 1/2" below the hole when the oil is cold.
Stick your pinky finger in and you should be able to
touch the oil.
Don't over fill, it will expend when hot to the bottom of
the fill hole.
dg

Denny - That transmission looks great! Did you rebuild that yourself?

Yes, I do all my own work, cept on the late model stuff, which
I'm not equipped to trouble shoot....yet.
dg

JackAttack,

I know this has been a controversial subject in previous posts concerning gear lubricant for the 3 speed and 4 speed transmissions in these AD trucks. It has been my experience, especially with the old cast iron Muncie 3 speed transmissions that have cone type synchronizers that the GL-5 80W-90 gear oil is too slick causing the synchonizers to slip thus causing double clutching for gear engagement. These sychonizers in turn wear much faster. A few years ago I spoke with a GM line worker that worked for GM Atlanta, GA. plant in the late 50's. He was the person that convinced me that the straight 90 weight mineral oil based gear lubricant was the way to go. He told me that GM used this mineral based gear lubricant up until the mid-seventies, even in the Muncie M-21 and M-22 4 speed transmissions. He explained that the mineral oil based gear lubricant had a higher level of ash which in turn caused these synchonizers to slow and engage as designed, thus eliminating double clutching. I purchased my straight 90 weight gear lubricant at NAPA. You have to ask the guy behind the counter and they usually have it in the back in "dust" covered gallon jugs. This gear lubricant is also used for antique tractors. I had a this problem with an old Muncie 3 speed in my '58 Chevrolet Biscayne. Problem solved after changing to straight 90 weight mineral based gear lubricant. Just letting you know my experience. Take it for what it's worth, it your choice.

Tim

SAE 90W mineral oil is what the shop manual calls for use in transmission.
SAE 90W multipurpose in rear axle, (and as a alternative to the mineral oil in transmission.
80W for extreme cold locations.

For those seeking proper gear oil for the SM420, today I made the drive to several sources and came up with two places that carry/stock or can readily get the SAE90 Mineral oil required.
Price was about the same 18-20 dollars a gallon.
NAPA has the following:
[img]http://www.spicetraders.com/photos/sae90mineral2.jpg[/img]
I bought a gallon today.
While over at Tractor Supply Stores I found this:
[img]http://www.spicetraders.com/photos/sae90mineral.jpg[/img]

48X3600,

That's it! The stuff works and I swear by it! If you use the factory specified transmission lubricant, the tranny will last a lot longer! $18 -$20 a gallon is "cheap insurance."

Tim

Warren Oil still makes 90W mineral oil. Page 38 is the mineral oil. I guess I'm selling it too cheap at $9 a gallon.

http://www.warrenoil.com/catalogs/warren_catalog.pdf

Thats all we used in all of our transmissions until synthetic came out. Instead of our transmissions requiring bearings every 100,000 miles, we now go well over a million miles. I use it in everything. Denny got rid of the synthetic because it is so slick that his synchros didn't grab. I don't notice that because I always shift without a clutch anyway.

I sell Warren's Type A ATF for $2.49 a quart. I read in a lot of forums that type A is no longer available. I just got through placing an order for a pallet which is 84 cases of 24.

I will add the Warren catalog to my folder, in case the NAPA and Tractor supply closes. We just went from three NAPAs to one. And the way store in town (and around the state) keep closing, mail order my be the only way.

Just FYI

GM switched to Synchromesh in manual transmissions about a dozen years ago.

The modern GL5 oils were too harsh on brass synchros.

Anyway, Synchromesh might be a vialble alternative.

http://www.pennzoil.com/other-car-p...ear-oil/manual-transmission-fluid/#About
http://www.oreillyauto.com/site/c/d...h_N2281_-1_-1&pt=N2281&ppt=C0207