Oil for M41 gearbox and differential
 

I'm sure this has been asked (and answered) before, but I couldn't find a definitive answer on the forum in my searches. I am no expert on oils, so please bear with me.

I'd like to change the oil in the M41 gearbox and rear differential in my '68 Amazon. Driving in the UK all year round, so mostly around 10 deg C.

The green book states
gearbox (non overdrive): SAE 80 all year round "hypoid oil must not be used"
final drive: SAE 90

Can anyone recommend which oil to purchase for gearbox and final drive please? What are the best brands to get? Is there much of a difference between the more expensive vs the cheaper alternatives?
Many thanks for your advice.

gearbox oil
 

Hi
non overdrive box> SAE 90
overdrive box > 20W50 ENGINE OIL

Ta
Kassie

oil
 

As the green book states.
The M41 overdrive box oil is 20/50W the same as the engine. The overdrive uses a wet type clutch so use oil without slippery additives. I use the 20/50w oil in the green can at £15 for 5 litres at Wilkinsons. The guys at Overdrive Repair Services in Sheffield confirmed it was OK. They recommend changing the oil regularly to preserve the life of the unit. £15 worth of oil is sufficient for 2 changes.!!
On the overdrive there is a 3mm thick metal plate about 100mm x 50mm retained by 4 small hex screws. Remove this carefully to avoid damaging the cork gasket, clean the brass filter/basket and magnetic washers thoroughly at each oil change. The cork gaskets are readily available at ORS. Once the filter and oil has been changed,you may notice a difference how quickly the overdrive engages.

With SAE20W/50 engine oil in the gearbox & overdrive, would the oil additives such as detergents and dispersants and the like have any negative effect on internal components?

Thanks for your comments so far. I was asking about the gearbox without overdrive and the differential.
Thanks

Numbers game
 

Overdrive box is an M41. Non-o/d is an M40.

@Volvo_133...

What makes you say that the OD is of the wet clutch type?
It is not.

Hello nortNH,
the clutch inside the overdrive is a wet type clutch for shure.
If the car is equipped with OD or not, there is no difference between the clutch on the flywheel of the engine. And this is a dry type one.
Engine oil into the gearbox with OD, 20W50
GL5 SAE90 into the rear axle, hypiod oil must be used.
Regards, Kay

overdrive clutch
 

The cone clutch within the overdrive operates within the overdrive oil which is shared with the gearbox. Slippery additives can affect the overdrive clutch.

Has either of you actually dissembled one of these ODs and held the dry oil-free friction-surfaced cone clutch in your hand?
Have you ever looked at an explanation of how this unit works?
https://www.uniquecarsandparts.com.a...****_overdrive

The fluid in this unit is for lubrication and hydraulic control of the sliding cone clutch only; the friction surfaces are dry.
Hence a dry clutch.