[ROVERNET - UK] P4 & P5 gearbox and overdrive oil

S Manwell smanwell at sprintmail.com
Mon Oct 10 22:15:30 BST 2005

I have been thinking of trying a synthetic gearbox oil in the gearbox 
and overdrive of our 1960 P4 100.  I'm wondering what experience others 
have with these gearboxes, but here is what I have done and researched 
so far:

The car now has 20W50 engine oil in the combined gearbox and overdrive.  
The overdrive works fine and the gearbox shifts well when the car is 
cold. Once it is warm, it gets difficult to shift into 2nd and 3rd.  The 
car has about 40,000 miles on it, but the clutch was not fully 
disengaging for a few thousand miles before we got the car -- so, the 
sychros may be worn.  I'm not sure how I arrived at using engine oil in 
the gearbox in the first place, because in reading the owner's manuals 
for both the 100 and 3 litre MkII today, they call for 90W gear oil, in 
fact the oil recommendation for both the gearbox and the rear axle are 
lumped together in one column.   All of this seems to suggest that the 
100's gearbox might shift better with higher viscosity oil.

It's more complicated than that, though.  The late 3 litre workshop 
manual that covers MkI through III says that later overdrive gearboxes 
(MkIII and some MkII) have a separate oil system for the overdrive vs. 
the gearbox.  And, the oil recommendations in this manual call for 20W 
for the overdrive and 90W for the separate gearbox. 

The MkII and III recommendations seem to jive with the recommendations 
from this overdrive rebuilder:
(see the topic for June)
I think the long and short of his tests is that overdrives prefer thin 
oils like ATF and 30W.  He thinks that detergent engine oils can cause 
cavitation in the overdrive hydraulic pump and that 90W hypoid oil can 
cause the hydraulic pressure to be too high.  He doesn't sem to mind 
synthetic oil other than its propensity to leak. 

All of this has me thinking of trying Red Line MTL, aka "manual 
transmission lube".  I am comfortable putting this synthetic lube in the 
gearbox based my experience with it in my Saab 900 gearbox and what I 
have read about others using it in Saabs and other cars.  It also seems 
like it should be OK in the overdrive since its motor oil viscosity 
equivalent is SAE 5W/10W30 and its gear oil viscosity is 70W80 GL-4.   
Technical details on this oil are here:  

I have emailed Red Line to see what they recommend for overdrives, but 
so far I see no specific issue with MTL so far...

--Steve Manwell

More information about the rovernet mailing list