Thread: Fairey Overdrive on LT95 - Oil

Originally Posted by landrover dave

Lt95s have a gear oil pump driven off the layshaft. These oil pumps are a fibre ,not unlike the old holden red 6 cam gears, and the square drive shears off when heavy gear oil is used. To get to the pump you have to remove the box and take the front cover that the input shaft comes out of. Be very careful what oil you choose as friction modifiers can stuff up synchros so will a gl4 rated oil.

Even with the latest steel pump set you still have to use a 20w/50 or sae30 oil. The Fairey overdrive is designed to use thae same oil!

No it won't, although some GL5's can be aggressive towards yellow metals.

FWIW engine oils have the wrong friction modification for proper synchro engagement as their coeffecient of friction decreases with deceasing speed, the exact thing you don't want to happen and most engine oils are loaded with friction modifiers too.
I think you may be thinking of LSD friction modifiers which are a no-no in any synchro gearbox as their coefficient of friction decreases remarkably at high pressure/low speeds.


The only reason engine oils were used is that they were the only lubricants that were generally available at the time that spanned the required viscosity range. ie. wasn't too thick during a cold start to shear the pump drive and delivered adequate shift action with reasonable gear/bearing life.

Some engine oils work fine as they are heavily loaded with antiwear and extreme pressure additives (think diesel oils) but the experts (the blokes that blend the stuff) tell me a dedicated manual trans fluid will always outperform an engine oil in a manual gearbox.

I try and stay out of LT85/LT95 oil discussions as people just don't get SAE viscosity ranges and can't equate them to kinematic viscosities, but any modern 75W-90 manual trans fluid (eg Redline MT90) spans the same viscosity range as a 15W-40/15W-50 engine oil.

Fluids such as VMX, VMX-M and Syntrans 75W-85 are closer to 10W-30 engine oils.

This is an old chart as it doesn't list the newer SAE 110 gear oil viscosity range that slots between SAE 90 and SAE 140, but you'll get the idea.
It also doesn't show the low temp pumping pressures of the W grades.

I am staying with engine oil - works fine in our -5 mornings in winter, great in our 30+ summer mornings - gear change is great and the overdrive seems to like it with easy gear changes.

Plenty of good gear oils were around when the LT95 was designed yet landrover went for engine oil.

I am sticking to it.

Garry