Hitchcocks Forum

At work we have a surplus of EP85-140W gear box oil, I think this is from farm equipment etc, not sure why we’ve got so much but there we go. I’ve been using it in my 2008 Sixty-5 Enfield gear box but I have just been informed that I probably shouldn’t as it can damage some components. What do you guys think? Should I use it or not? If not what should I be using instead?

The usual reason why certain gear oils may damage some gearboxes is that hypoid oils can attack plain bearings. I don't know whether or not the iron barrel gearboxes have such bearings so I can't advise you. What I would say however is that anyone who has an old Moto Guzzi V50 or similar would definitely be interested in that oil, why is pretty expensive. It''s used in the final drive bevel box.

If anything such oil would be over specified for an Enfield gearbox, in any case what will "rot" plain bearings is molybdenum disulphide which is not present in that oil are far as I am aware.
(I don't think Enfield's as late as yours have plain bearings in the gearbox anyway.)

The extreme pressure (EP) additives used in such oil are usually based on sulphur phosphorus. This can discolour , and eventually start too corrode yellow metals in the gearbox (or anywhere else) it is used. This type of oil is designed for hypoid gear drives (offset spiral bevel) where there is a lot of sliding and potential metal to metal contact. The sulphur phosphorus additive helps to reduce wear caused by the sliding of the gear teeth. This oils viscosity range is very wide and can be anywhere from 315 cSt at 40 C to 775 cSt at 40 C and is heavier than a standard SAE 50 engine oil. Unless the bike specifically asks for such an oil I would avoid using it.

Thanks; I knew it isn't molybdenum disulphide that is in EP hypoid gear oils. Molyslip is the black stuff I used to add to the gearbox and final drive on my Moto-Guzzi (as recommended by M-G). If it were in EP 90 etc the oil wouldn't look golden or have the telltale pungent aroma (well, stink really) that that oil has. FWIW Morris oils sell a 50 weight oil (equivalent to 90 weight gear oil) specifically for old Brit bikes with gearboxes that have plain bearings. I use it in my pre-unit Tiger 100. Still leaks though...

Molybdenum disulphide (MoS2) is excellent for some situations, especially where sliding or incomplete rotations are present i.e. there is not much opportunity for a regular oil to be spread across the contact area and stay there. It shears in one direction (bit like a pencil lead) and this is what provides it lubricating ability. It is a 'solid' lubricant though and does not dissolve in the oil or grease so the size is important. One of the concerns using MoS2 is it's purity and the more expensive it is usually the higher the purity. One of the contaminants that is mined with MoS2 is silica (dirt); something which is definitely not wanted in a lubricant. I've seen some components with abrasive wear due to the application of MoS2 (most likely not very pure) and where it wasn't necessary to use, as a regular lubricant would do the job nicely. MoS2 is good in the right circumstances, but, use with care.

Back to one of the original questions. I don't know what oil is recommended for the gearbox, however, if it asks for an SAE 90 non EP gear oil you can use an industrial style gear oil, like Mobilgear 629 (ISO VG 150) or 630 (ISO VG 220) or Shell Omala 150 or 220 or something comparative. If you want something slightly more viscous you can use Mobilgrear 632 (ISO VG 320) or Shell Omala 320. These two are just at the top threshold of the SAE 90 grade range. You could also use a 50 grade engine oil (straight 50 .. not a multigrade) , as suggested by Rattlebattle as this sits in the middle of the SAE 90 grade range. Something like Mobil Delvac 1350 or similar has been used for years in Road Ranger gearboxes (heavy vehicles) because of the yellow metal staining concern when using an EP style oil. The only concern with using an engine oil is that it contains a lot of additives not required for gears, including detergent/dispersant which can cause the oil to emulsify easily in the presence of water. Regular gears oils tend to separate from water so they can be drained off if necessary.
By the way the EP additive gives the oil it's distinctive smell - I guess due to the presence of sulphur.

Thanks for that; most informative.