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80W-90 in 4-speed?
#1372434 Fri Aug 07 2020 02:09 PM
Joined: Sep 2009
Posts: 314
Shop Shark
Good morning Stovebolters.

I've had a hard time finding 90 weight mineral oil here locally, and wondered if 80W-90 is ok to run in my SM-420 4-speed. The closest thing I've found that would probably work is at Tractor Supply; 90 weight mineral "Traveler" brand Ford tractor transmission oil. This would probably work just fine wouldn't it?

1953 Chevrolet 3100
261 / 4-speed / 4:11 / Commercial Red

Re: 80W-90 in 4-speed?
Green_98 #1372445 Fri Aug 07 2020 03:35 PM
Joined: Jul 2014
Posts: 623
Shop Shark
Yes , the Tractor supply mineral oil is the correct oil to use as it is the mineral oil that many on this site swear by. Others more knowledgeable would tell you that the some of the newer oils don't work well with the brass synchros.

1954 Chevy 3604 - A work in progress...

" You can't keep dancing with the Devil and wonder why you're still in hell"
Re: 80W-90 in 4-speed?
Green_98 #1372448 Fri Aug 07 2020 03:48 PM
Joined: Feb 2004
Posts: 21,005
The SM-420 is a pretty forgiving transmission, but it's got big, heavy gears with enough inertia that the synchronizers have a problem matching gear speeds when shifting. Stay away from the super-slippery synthetic lubes that might not let them do their thing. The Tractor Supply 90 weight mineral oil sounds like the best choice, and it's probably a lot less costly than the fancy Moose Juice to boot! Use it in the transmission only- - - - -it won't have the hypoid lube additives the rear end needs. The 80W-90 would be a better choice there, or any general purpose hypoid gear lube.

The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: 80W-90 in 4-speed?
Green_98 #1372457 Fri Aug 07 2020 04:37 PM
Joined: Sep 2009
Posts: 314
Shop Shark
Makes sense, thanks Jerry. Rear diff flush is coming up, so I'll pick up some 80w-90 as well. It's purring like a kitten smile

1953 Chevrolet 3100
261 / 4-speed / 4:11 / Commercial Red

Re: 80W-90 in 4-speed?
Green_98 #1372470 Fri Aug 07 2020 05:46 PM
Joined: May 2001
Posts: 1,522
Shop Shark
According to my '48 owner's manual, a single number like 90 (w/o the W) is a defined viscosity at 130 deg F. The W indicates a viscosity scale determined at 0 deg. The 80W-90 gives the extra cold info but it seems to me the 90 mineral oil has a W number that could be measured but it just isn't given.

I mean, 20W-20 used to really puzzle me.

I haven't bothered to Google the modern definitions and I'll just rely on my owner's manual. I use the 80W-90.

1948 3/4-Ton 5-Window Flatbed Chevrolet []

28 Years of Daily Driving but now on hiatus. With a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.
Re: 80W-90 in 4-speed?
Green_98 #1372493 Fri Aug 07 2020 09:21 PM
Joined: May 2015
Posts: 2,759
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
If they tested straight 90 weight gear oil at 0 deg, it would probably come out at about 140W-90 (yes, you read that right wink ).
So 20W-20 would be the same viscosity at both 0F and 130F.
I ran 140 weight in my Model A (Straight Tooth non-synchronized) tranny. Then I got clever and put some STP oil treatment in it. One morning it was in the 30's and I nearly bent the shift lever trying to get it into gear.

First car '29 Ford Special Coupe
#2 - '29 Ford pickup restored from the ground up.
Newest Project - 51 Chevy 3100 work truck. Photos []
Busting rust since the mid-60's
Re: 80W-90 in 4-speed?
Green_98 #1372916 Tue Aug 11 2020 07:40 AM
Joined: Oct 2018
Posts: 67
Wrench Fetcher
Not sure exactly what specific 80W-90 oil you are referencing, but many of the gear oils on the shelves of auto parts stores (at least the ones by me) contain an API service rating of either GL-4, or GL-5.

While the EP (Extreme Pressure) additives contained in GL-4/5 rated gear oils are great for reducing wear in hypoid gear sets, most are not particularly friendly to any yellow metal components in the transmission - i.e. synchronizer blocker rings....

The following link has a lot of interesting information on this topic:

Gear Oil []

While there are exceptions, many of the data sheets for these oils contain similar warnings.

At one point, I might have leaned toward finding one of those "exceptions", but after looking at some of the spec. sheets for some of the plain old 90 weight GL-1 oils, it looks like the manufacturers are putting in a pretty substantial additive package (corrosion inhibitors, anti foaming, etc.).

That said, here is the data sheet for the "Tractor Supply Traveler Ford Tractor 90 weight oil".

Ford Tractor Oil []

While the top of the data sheet says "API Service GL-1" (which is good for you) , further down the page it says: "Performance Standards: GL-4" - which is confusing to me. If it meets GL-4 performance standards, does it include any EP additives?

Perhaps you could shoot them an email telling them your application and see what they say. The fact that many on this site are already using it without any issues is obviously good sign, but it would not hurt to double check.

Best Regards....

Re: 80W-90 in 4-speed?
Green_98 #1373208 Thu Aug 13 2020 04:04 PM
Joined: Jan 2002
Posts: 2,096
Shop Shark
Besides Tractor Supply, NAPA also carries 90w Mineral gear oil API GL-1. Besides, do you really want to use a oil designed for Fjord Tractors in your Chevy gear box ? Ewww! LOL

Last edited by cmayna; Thu Aug 13 2020 04:56 PM.

Craig's '50 Chevy 3100 5 window
My truck
If I'm not working on my truck or VW camper, I'm fishing with the wife or smoking Salmon.
Re: 80W-90 in 4-speed?
Green_98 #1373230 Thu Aug 13 2020 06:44 PM
Joined: Mar 2019
Posts: 1,024
Shop Shark
PSC.Com carries 5gal buckets of 90w gear lube. Thats what i use. And its nice to have it available at home when you decide to change trans or differential lube on an off time.

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