The Stovebolt Site Forums The Shop Area Driveline 49 GMC original - fluid equivalents

Hey all...very cool group here. I have a bit of a tech question and I'm hoping you current owners can help me out. I've recently purchased a "restored to factory" 1949 GMC 3/4 ton. Every component in it is again, restored to factory spec but the previous owner doesn't know what was used during the restoration. That being said, fluid equivalents has left me scratching my head a bit despite being very mechanically inclined. For example..."heavy duty hydraulic brake fluid" as was called in the factory service manual is what now? DOT3, 4 or 5? I'd guess 3 but not sure. Also...shock absorber oil...what type and weight? Hypoid gear oil for differential...80w90, 90w140? And even transmission...dont think it would take a hypoid oil but...what weight? The factory service manual states nothing other than "hypoid oil, or hyd brake fluid, etc". In many of the others...it's a 20W engine oil to lube pivots, and 50W for engine which is no issue. Any help or advice would be greatly appreciated.

Good questions- - - - -and you're going to get a bunch of sometimes very different answers from the stovebolt crew. Opinions are like "noses"- - - -everybody's got one, and some of them are pretty smelly at times!

Brake fluid- - - -DOT 3 is good, DOT 4 is better. DOT 4 was developed to handle the higher temperatures encountered in disc brake applications, but either one will be OK for use in a stovebolt truck. RUN- - - -do not walk away from DOT 5- - - - -it's silicone based, and does not play well with other fluids, and possibly some of the rubber parts in a stovebolt brake system.

Gear lube- - - -straight 90 weight mineral oil for the transmission, Hypoid gear lube for the rear end. 90 weight in the rear end was the original specification, but multi-weight will work. Synthetic hypoid gear lube in a stovebolt transmission- - - -particularly the SM-420 4 speed, can cause gear grinding because it's too slippery to let the synchronizer blocker rings work properly.

Engine oil- - - -lots of opinions here, with everyone believing their prejudices are justified. I prefer using 15W-40 Diesel rated oil in everything I run from modern high-mileage vehicles to stovebolts, to my Harley and all the small engines on the farm. No oil-related engine failures in 40-something years of doing that. Don't try that on a brand new modern engine- - - -that's where the 5W-30 and 0W-30 oils are needed. Once even those engines get 100K + miles, a thicker oil is a good idea because of increased clearances from normal wear. 50 weight oil is for racing engines- - - - -it's foolish to run it in a street engine of any kind.
Jerry

Thank you Jerry. Funny thing...go to a typical parts store...GL5 8w90 and 90w140 is all they have. Only had 6 quarts of sae30. And all DOT3 synthetic. So heres what I've come to and please see if you agree...

Brakes std DOT3
Diff GL4 80w90 Hypoid mineral
Trans GL4 80w90 mineral
Engine SAE 30
Shock Absorbers 10 or 15 weight shock/fork oil

All that's left is steering gear box. Your thoughts?

The steering gear box take a NGLI 00 or semi-fluid grease. Contact Drdoug on here. He can fix you up.

If you've got a TSC (Tractor Supply Co.) store in your neighborhood, they have a better selection of fluids than the average "friendly local auto parts store"- - - -which you'll find abbreviated "FLAPS" on this forum. The lever-arm shocks will work with virtually any lightweight oil such as power steering fluid. NAPA also has a better selection of gear lubes than most FLAPS since they service commercial customers like trucking companies, but you'll need to find a counter man who is willing to do a little research. The 20-something mouth-breather computer jockey at the front counter of some parts stores thinks it's an imposition on his important text messaging time to actually look up something in a paper catalog or listen to an oddball request from the customer whose purchases are generating his paycheck!
Jerry