Recommend Modern oil for M41 gearbox, UK climate?
 

Hi all,
Got the 1971 P1800E after some engine and electrical work. The engineer advises a full change of gearbox fluid ASAP.

I am getting fi ding a few choices on the forums; all suggest low friction engine o of SAE 30, 10W40 or 20W50 spec.

Intention is to add correct high detergent oil, run 500 miles fast road, then do a hot change. Twice.

The overdrive works flawlessly, I just want to keep it all happy.

Warmest Regards to all

Hi, oft discussed topic. The Volvo spec by the manual is either 30w or 20w40. 30w may be too high viscosity below about 5 deg C. So a multi grade would be the better choice.

Not sure what you mean by “low friction”?, synthetic motor oil not recommended for use in these gearboxes. Interferes with the operation of the OD.

“High detergent” not required , unlike the engine which produces contaminates as a byproduct of combustion, a gearbox runs “clean”. In fact some earlier models of the 1800/M41 specified “non detergent” 30 weight motor oil. I suspect the non detergent spec was dropped due to the scarcity of non detergent oil.

There are several specialty gearbox oils available , the only one I have experience with is Redline MTL, specifically designed for manual transmissions and I find an improvement over regular motor oil, shifting is improved.

20w40 oil is not very common , 20w50 is commonly used in M41’s, so that would be your best bet.

While your at it, you might consider changing the differential oil, that’s often ignored. A HYPOID gear oil is required for that. SAE 90 is specified , however a multi grade like 85w90 is fine, and commonly available.

To reenforce C1800's advice, the 1971 owner's manual says SAE 30 or SAE 20W40 engine oil. Red Line MTL is also a good (expensive) choice if your synchros are getting a little tired. I use Red Line in my M41.

You will find references to using SAE 80 or 90 gear oil (non EP5). SAE 90 gear oil is about the same viscosity as SAE 30 engine oil. I recall that Red Line MTL is actual specified as a gear oil with an SAE 80 or 90 viscosity.

If it has been a long time since a gearbox fluid change, do a complete drain by removing the cover plate on the OD (in addition to the transmission drain plug). Have a new cover gasket on hand because the old one will probably not reseal and will likely tear on removal.

https://www.classicvolvoparts.co.uk/...+%283502464%29

There is a filter screen under the cover that that can be easily removed and cleaned. There is also a circular filter under a cover plug (need a pin wrench) that can usually be cleaned.

https://www.classicvolvoparts.co.uk/...SHER+380730%2E

Excellent!
I knew this place wouldn't fail me.

I will hot-drain just the Gearbox and refill with 20W50, run it about 500 miles and do another hot-drain which includes draining and cleaning the OD chambers. I will look at the REDLINE MTL, failing that a 20W-50.

Thanks for reminding me re the differential, there is a further note from the engineer re this too, you saved me a second post. Would it be sensible to add a friction additive like molybdenum to the diff during the change, or not?

Just for your knowledge, the OPIE UK site advises "This product has recently changed viscosity grade from 70w-80 to 75w-80." which may be pertinent for you two.

Warmest Regards
U

First off, be aware that the gear box and the OD share a common oil supply. When you drain the gear box using the gear box drain plug you drain 'most' of the oil out of the OD because they share a common connection through that center joining thingy (non technical term). A little bit of oil always seems to stay in the OD, hence my suggestion to pull the cover plate and cleaning the filters is never a bad thing. You can probably drain the OD (and gear box) by removing just the OD cover plate; but, I expect this will be painfully slow. So, no need for two separate activities.

Unlike engine oil, gear box oil does not get contaminated by combustion by-products. A single fluid change should do the trick and reduce your environmental impact. As an observation, where I live the gear box does not really get 'hot' from operation, particularly at this time of year. If you want a 'hot' change you may need a heated garage or an external heater - or wait until June.

I would not bother with additives for the differential. If you want to spend a little more there are synthetic gear oils. You will probably have trouble finding a straight SAE 80 gear oil and may have to go with an SAE 80W90. Get a GL5 rated oil and you are good to go. Do not get a gear oil with the friction modifiers intended for limited slip differentials (unless you car was the rare car that came with or had the LSD retrofitted). As a bonus, the differential oil is the same oil used for the steering box so you can use any left over to top up the steering if required.

Redline also makes an excellent Hypoid/GL5 gear oil for differentials in 75W90 weight. Given the original Volvo spec - 90 weight oil for the diff, 75W90 would be correct.

I like the classic oil scene (if only for the nice cans they sell the stuff in!)

Here in the Benelux countries I buy stuff made by Kroon because it is widely available. In the UK I'd probably be a Classic Castrol boy (for an equivalent reason also given in brackets above)

I vaguely remember cans. Unless you had the spout tool thing, you had to whack a couple of holes in the top with a screwdriver and use the high tech wood screw in the hole to seal it if you didn't use the full can. Nostalgia is OK; but, I much prefer the 5 l plastic bottles with the screw on caps. Provides a convenient container for taking the old oil and filter back to the vendor (which they are obligated to accept in my province). Keeps the old oil out of the landfill or worse, the aquifer.

Completely agree with the comments about responsible disposal. Here in NL the "Chemokar" is happy to take oil in any original packaging. They have told me they prefer it that way as it helps them decide which of their vats the contents need to go. It is a nice service - they come to your house here to collect.

Those cans had many uses. Back in the days, I turned one of them into a guitar.

https://www.youtube.com/watch?v=bXtNDQZdw0s

Can’t do that with a plastic one.