Correct Gear Oil+

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Terlingueno

Fictional Western Sage
Joined
Dec 29, 2020
Posts
465
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1,664
Location
Far West Texas
First Name
Skeeter
Truck Year
1985
Truck Model
K20 Suburban
Engine Size
350/SM465
I went to the local community market/flea market this morning to get some eggs and look at what people had. Several local guys, a couple brothers, dad and grampa (they are kinda new around here). They struck up a conversation about fixin on stuff. They also have quite a few old trucks/cars, square bodies, several 70's-80's GM cars. They tell people they are 'mechanics' but they seem to have problems keeping things on the road. So they were eyeballing my Suburban and one noticed the SM465 shifter. I told them I was fixin to change out the trans and both diffs fluids pretty soon since it's overdue for changing.

Grampa and dad started right in with the 'gotta get the right gear oil for these old trucks' schpiel. I responded that I had enough GL-5 from VP to get it done. Immediately they got loud and irritated and said 'you don't know nothin you gotta use GL-4 in that truck'. I responded with 'well I have always used GL-5 and it's been good since 1994 when I got the Suburban'. You would think I kicked their dog they got so animated about 'the wrong gear oil'. This went on for about 5 minutes and I said 'I've got the owners manual and maintenance schedule that came with the truck in the glove box and it states GL-5'. They pretty much called me a liar so I went to the glove box and showed them this:

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Grampa and dad started yammering that that document is incorrect and I should use GL-4. I then told them that I also had the GM shop manual and it states the same thing. Dad starts in with I was a relying on the wrong information. Since it is only a 10 minute drive from my place to the market, I called my gal and told her I would buy her a cheeseburger for lunch if she would bring the shop manuals so I could show 'the mechanics' an actual re-printed GM document. Even after seeing the glovebox document and shop manual they insisted I was gonna ruin my transmission and differentials if I used GL-5. I said 'have a nice day', took my eggs and walked calmly away and we went to the cheeseburger place...

So for the record, here is the applicable shop manual pics:
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You must be registered for see images attach
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
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10,034
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19,298
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
The big deal with GL4 is it is safe for something with brass synchronized. They'd have been right if we were talking about a brass synchro transmission. The transfer case doesn't know the difference. The pressure additives in some gear oils are too sticky, they stick to the brass and continually strip away minut bits if brass as the gear oil gets pushed out. So it wears out the synchros.
 

Smtty58

Junior Member
Joined
Mar 19, 2026
Posts
21
Reaction score
18
Location
Mass.
First Name
Lyle
Truck Year
1990
Truck Model
R3500 Crewcab
Engine Size
350
I thought 2-4 have synchros?
 

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