75W/90 Gear Oil is the same as 5W20 oil. NOT

Joined
Nov 23, 2015
Messages
1,962
Location
US
Not that it's overly relevant to the topic at hand at what I assume powertrain use, but within British car communities among people who prefer a thicker carburetor oil, as to whether 20W-50 engine oil, xW-90 gear oil, or SAE 30 is most appropriate. Carbs tend to run relatively cool thanks to the vacuum/fuel atomization, and I've measured 120-130ºF when properly warm but not heat-soaked with an IR thermometer. I know there's some variance, but at those temperatures they all look pretty similar to me. Since the only purpose of this oil is to slow the rise of this piston(it doesn't really lubricate anything) friction additives and detergents don't matter. Dampening the piston movement replaces the accelerator pump in this type of carb, which can otherwise feel a bit less responsive than a fixed venturi carb w/a pump, and the idea behind thicker oil is that it will enhance this effect. I've actually gone as high as SAE 50, although it's hard enough to find and another oil to keep on hand that I quit bothering and went back to either engine oil or gear oil(gear oil often wins just because the nozzle makes it easier to not make a mess).
 

MolaKule

Staff member
Joined
Jun 5, 2002
Messages
23,072
Location
Iowegia - USA
Originally Posted by bunnspecial
Not that it's overly relevant to the topic at hand at what I assume powertrain use, but within British car communities among people who prefer a thicker carburetor oil,
What is meant by carburetor oil?
 
Joined
Nov 23, 2015
Messages
1,962
Location
US
Originally Posted by MolaKule
Originally Posted by bunnspecial
Not that it's overly relevant to the topic at hand at what I assume powertrain use, but within British car communities among people who prefer a thicker carburetor oil,
What is meant by carburetor oil?
It is used in a dashpot that slows the rise of the piston in this particular style of variable venturi/constant depression carburetor. [Linked Image] The constant depression effect is managed by the piston rising up and down to vary the size of the venturi(and consequently change the size of the jet by controlling how far a tapered needle is set into it). The only time the vacuum in the throat is NOT constant is on acceleration, when the upward piston movement is retarded by oil in the dashpot. This increases the vacuum, which provides for the desired richer mixture on acceleration. Viscosity of the oil used affects how quickly the piston responds.
 
Joined
May 6, 2005
Messages
8,884
Location
San Francisco Bay Area
Originally Posted by ARCOgraphite
I don't understand the purpose of this thread. As Mola referenced, a GL-4 or GL-5 gear oil is additised much differently than engine oil. Substituting either for the other would cause imminent desctruction.
Old thread I know, but older Honda manual transmissions used to specify 10W-30 or 10W-40 motor oil as a gear lube. Really. It was really odd too because the owner's manual would always specify an older API rating. Back when SH was the standard, it would say SF or SG. I called up Acura customer service once and all I got was that I should only use SF or SG, even they'd been superseded. They did quickly change to Honda MTF and said it was OK in any application where Honda previously specified motor oil in the transmission. Red Line had their MTL, which they claimed to be a 75W80 GL-4 gear lube suitable for 10W-30 motor oil replacement in manual transmissions.
 
Joined
Jan 2, 2019
Messages
1
Location
Florida
Old thread I know, but older Honda manual transmissions used to specify 10W-30 or 10W-40 motor oil as a gear lube. Really. It was really odd too because the owner's manual would always specify an older API rating. Back when SH was the standard, it would say SF or SG. I called up Acura customer service once and all I got was that I should only use SF or SG, even they'd been superseded. They did quickly change to Honda MTF and said it was OK in any application where Honda previously specified motor oil in the transmission.
...
Red Line had their MTL, which they claimed to be a 75W80 GL-4 gear lube suitable for 10W-30 motor oil replacement in manual transmissions.[/html]
My vehicle specifies 75w-85 GL4 transmission oil. I could not find it locally and called Royal Purple to see if their 75w-90 GL5 would work for my application since it was available locally and indicated that it was non corrosive to yellow metals. The tech told me that it would in all likelihood be fine. He indicated that it would not harm the transmission, but that it could potentially cause cold shifting issues. He strongly recommended that I use instead "Royal Purple HPS Synthetic Motor Oil 10W-40". He said that only the HPS version of the motor oil would be compatible due to the additives package. To boot, it is less than half the price per quart.

I think that there are fewer differences in oils than we are led to believe. Like any other industry, a manufacturer wants to produce as few different formulations as possible to maximize profit. Branding is often the only real difference.
 
Joined
May 6, 2005
Messages
8,884
Location
San Francisco Bay Area
I think that there are fewer differences in oils than we are led to believe. Like any other industry, a manufacturer wants to produce as few different formulations as possible to maximize profit. Branding is often the only real difference.
Not sure about that. It's so easy for any company just to buy an additive package from Lubrizol, Oronite, Infineum, etc. and match it to an approved base lubricant.
 
Joined
Aug 2, 2018
Messages
1,550
You’re comparing apples to oranges. When I need gear oil I buy gear oil when I need motor oil I buy it. I would never even talk about the two in the same conversation
 
Joined
Jun 23, 2005
Messages
3,508
Location
Millbrae, CA
Originally Posted by motor_oil_madman
a 75w90 is about the equivalent of a 50 weight, but gear oil doesn't have the same additives engine oil does.
uh no.. no its not.

sorry wrong they are same vis at 100C BUT yes addtives are different
 
Joined
Aug 23, 2010
Messages
3,957
Location
pa
old posts often present new or unread elsewhere info for others + its great that older posts are viewable-sometimes!!
 
Top