[Mgs] Overdrive Diagram

[PaulHunt73]

Saying it doesn't need detergents (in this post) is completely different to saying 'it needs to be non-detergent' which is what you said earlier, and that is what I was questioning.  MGA recommendation was apparently (https://mgaguru.com/mgtech/gearbox/gt107.htm) for 30wt non-detergent as that was commonly available at the time.  All the detergent does is carry any particles around until they are filtered out by the OD (if fitted).  It doesn't foam as is often claimed, dish washing liquid foams from additives deliberately there to make it do that, who would buy a detergent that didn't even if it was as effective as cleaning dishes?  My Laycock manual doesn't specify what oil should be used, that is left up to the manufacturer of the vehicle and how it is intended to be used.  All it emphasises is that friction reducing additives should not on any account be used.

Saying the multigrade properties of engine oil break down over time in a gearbox may well be true, but it's a giant leap from there to say that means 30 wt straight oil is somehow 'better'.

Do you have a reference for earlier models needing something other than engine oil of 20W/50?  My period WSM and glovebox handbook state 20W/50, although for temperate climates it also includes 10W/40 and 10W/50.  Temperature ranges below that EXCLUDE 20W/50, and there are no different recommendations for 'hot' climates i.e. specifying it HAS to be 20W/50 and nothing else.

The V8 gearbox recommendation has always been SAE90 gear oil for temperate climates, SAE80 for colder, and I have a BL document somewhere that states this for the 4-cylinder as well (although according to the link above it shouldn't be used in 3-synch gearboxes as they have brass components), although others say that was rescinded.  That's not a recommendation to put it in an auto box of course, any more than not using it in auto boxes is a statement of fact that you can't use it in overdrives, which is another giant leap to nowhere.  Whilst the gearing arrangements use the same principle, the control is completely different.

My understanding is that it is only very early non-OD gearboxes that were splash fed, and only 1st gear at that, the others fed by a helical screw pump, and all four gears are pump-fed on the 4-synch - lots of info here https://mgaguru.com/mgtech/gearbox/gt201.htm.  That's regardless of whether overdrive is fitted or not, although they share the same oil the lubrication of each is completely separate.  Originally that source recommended not towing long distances with the rear wheels on the ground, but he says since then he has towed his 150 miles with no problems - but the oil level does need to be correct.
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  The factory manuals specify “engine oil” for both overdrive and non-overdrive transmissions.


  If you analyze that recommendation, you would conclude that the key characteristic they are concerned about is viscosity. Analysis follows:


  Transmission oil does not have to deal with combustion blow-by or high heat, so it doesn’t need detergents or protection from thermal breakdown. And my personal take on it (though this is apparently highly controversial) is that for transmission oil it is better to use a single-grade oil, as multi-grade oils break down into the single-grade stock over time, and one does not change the transmission oil very often compared to engine oil (or ever, IME). It’s not that multi-grade would hurt; it is that its benefits, if any, won’t last, and the single-grade base stock is equivalent to the lightest grade in the rating; i.e. 20W50 breaks down into 20W.


  But some people will fly off the handle at the suggestion that 20W50 is not ideal. Their logic goes something like this: “the manual says ‘engine oil’ for the transmission; for my climate it recommends 20W50 engine oil for the engine, therefore 20W50 is the only correct oil for the transmission.” I believe it is only the later model MGs that have this particular recommendation (20W50) in the drivers handbook.


  Anyway, backing up a paragraph or two, to my mind there is a significant difference between the standard transmission, which uses oil for splash lubrication only, and the OD transmission, which uses oil as hydraulic fluid with a planetary gearset, much like an automatic transmission. Automatic transmissions are extremely sensitive to viscosity — I don't think you would have much luck with 90W gear oil in an auto trans.
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