I recently had cause to wonder about the same things because my gearbox used to feel like treacle on cold mornings, and gears changed very well when warmed up. But after about 5000 miles, the changes could not be made as smoothly at higher speeds, and the treacly feeling when cold was gone. I had Snyders oil and 5% Molykote M55 suspension in it. I like MoS2 lubricant because it quiets my slightly noisy worn gears and acts as a boundary lubricant under severe conditions.
Anyway, I drained the oil and did a few quick tests in my laboratory at my company with stuff I had to hand and came up with the idea that I needed a lowish viscosity at lower temperatures, and something higher than usual at working temps.
This requires viscosity index polymers and I had some Moreys oil stabiliser to hand, plus the Snyders Transmission oil, and the MoS2 suspension, so I mixed 20% Moreys, 3.8% Molykote M55 and 76.2% Snyders. I am now running on this and gear changes are back to very smooth and easy, with a treacly feeling when cold.
Then I saw this thread, so went and got some Lucas Hub Oil, 85W-140 gear oil (I could not get the lighter oil H L Chauvin mentioned here in NZ) and Lucas Heavy Duty Oil Stabiliser.
Because I am leaving for Marquette very soon, I did not have time to set up proper API tests, so I just tested in a water bath at 20 and 25 Deg C and an oven at 70 deg C – because we run both constantly in our lab for polyester resin testing.
I reckon the running temp of my gearbox on the motorway is about 50 deg C, but this is a guess and I will measure it when I get back from the USA. The higher the oil viscosity, the hotter the box will run and this will, in turn, reduce the running viscosity of the oil until a happy balance is achieved.
The results are; Snyder Trans oil =A; Snyder/Moreys/MoS2 blend as above=B; Lucas 85W-140 gear oil=C; Lucas Heavy Duty oil Stabiliser=D; 50/50 Lucas C and D=E; Lucas hub oil=F. Results are centipoises measured on a Brookfield viscometer. Multiply by the oils SG (about0.9) to get centistokes
Temp. A B C D E F
20Deg C 3920 ? 1330 1112 2280 7720
25Deg C 2800 3900 2550 ? ? ?
70Deg C 194 210 72 340 180 268
This was a very quick look, so all the results are not there, but they show how a polymeric viscosity improver like Moreys and Lucas can lower viscosity loss at higher temps.
Lucas Hub oil looks good in the table, but it is interesting Lucas themselves advised against it in another post. The Lucas 50/50 blend looks OK too, and the blend B I am using also holds viscosity well.
None of this is a recommendation, because I know nothing about what is in the components. It could be that the Snyder oil in blend B already contains a lot of VI improver and I have added too much now. This is why I put in MoS2, which acts as a boundary lubricant where lubricant failure conditions could be approached. My gearbox is a bit quieter with the Moly than without too.
Plus it says nothing about foaming and other properties, and my shear-thinned old oil certainly foamed.
The working conditions in a straight-cut box are a lot less demanding than a hypoid box, and it is reported on this forum that straight STP, and steam cylinder oil worked fine for many years, so I suspect any of the above would perform OK in a Model A
Apologies for the long post. But it may prompt more input and experiences to help us all
SAJ in NZ
