Syncro Gearbox/rear end oils

I see a lot of posts on this, but most seem focused on non-syncro Moss box, or have a lot of conflicting info, so I’m hoping for some clarification.

I have a '66 full syncro manual trans with overdrive. The shop manual suggests 30w or 20w/30, but not syncro, Moss, etc. and 90w for the rear end. There seem to be a lot of recommendations for synthetic oils like Royal Purple (MaxGear or SyncroMax) and Redline MT90 for the gearbox. Pricey for sure, but will probably not need to change these fluids for a looooong time, so might as well get the best.

Any thoughts? RP SyncroMax “is recommended for manual transmissions that specify an automatic transmission fluid or other light-viscosity oil” and 30w doesn’t seem lightweight to me. MaxGear “is recommended for truck and automotive differentials, …or in rear axles or manual transmissions requiring an API fluid.” Neither list a viscosity.

The correct oil for the o/d box is a difficult question to answer, Hypoid 80W90 or 20w50 or SAE30 or ATF or…
The overdrive and synchros seem to be the deciding factors. I went by the manual because I always try that first.
For the final drive the main question is if it has limited slip, and if the oil is good for hypoid gears which have high shear. And that it doesn’t have a sulfur content that eats brass, maybe.

I had the fortune to get some direct advice from Rich Michell- author of “which oil”, and a petroleum engineer with an interest in classic cars himself

he shed some light into the technology changes in these oils over 50 years with regards friction modifiers

He concluded that a GL4 was the closest to the original for the g/b and o/d

He found some Ampol literature from the 1980’s in which a GL4 oil was the “equivalent”

I put a few thousand miles on my 420 all synchro O.D box with good results. I used Redline 90.
I used GL4 gear oil in differential with limited slip additive

I went by the manual, too, but there have been advancements since then, and it does not specify full syncro or not. As for the rear, being a Mk2, I don’t think it has limited slip, but I haven’t confirmed that.

The three MK2’s that I owned all had limited slip differentials.

There is a test for LSD/Posi and it involves lifting the car and spinning one wheel. I believe the other wheel should turn in the same direction . Someone please chime in if I am wrong.

That’s correct, Gerard. I just haven’t gotten around to doing that yet. I just thought all Mk2s were not LSD. The shop manual only specifies 1 type of oil and doesn’t mention type of axle

Yes- my integration of the advice that ive had (from excellent sources - see earlier posts in this thread) is to use a GL4 75w90.
Here in Australia, there is an expensive fully synthetic Penrite product. Ive also found another brand with a mineral/semi synthetic oil that meets the spec (Hi-Tech oils)

Finally got the rear in the air. Mine spin in opposite directions, so not LSD. Maybe because it was a later model (66). Oh well.

If you ever decide you want LSD axle or just the workings, let me know. I have a spare.

You can also go to a 4X4 truck shop who are very skilled at this sort of work. If you so desire you could go for a lower or higher differential ratio. I have a 3.73 with OD so I am at 3000 at 70 mph. I have thought about a 3.54 for better fuel mileage on trips.