Seen mention of 4W40 engine oil used for the oil bath (lubriction) but the viscosity does seem a bit high compared to the recommended 15wt fork oil. Looked up the specs for the transmission fluid I use in my car and it actually has very similar viscosity.

Total BV 75W80: http://www.total-lubricants.ca/uploads/tx_listingpdf/EN/AUTOMOTIVE%20ENGINE%20OILS/Transmission%20fluid/TOTAL%20TRANSMISSION%20BV%2075W-80%20PIB.pdf

Is is the only stuff I trust in Peugeot and Citroen gearboxes and does a very good job. Anyone tried this grade for fork lube?

Also do I really need to use a grease on the seals? What to use with transmission oil? I was actually thinking about Mintex Cera-tec which is silicone based and safe for use on rubber seals and has no solid particles.

4W40?
never seen that 10w40 more likely

even so the viscosity of the oil when cold is the number before the W so the engine oil is less viscous than the fork oil

oops was late. I meant 5W40. 10W40 would be a bit thick and would normally be semi synth eg Total Quartz 7000.

It’s probably a bit more complicated than just matching the viscosity. Any reason not to buy fork oil? It’s not expensive and designed for the job.

fork oil for dampers yes. But this is for lubrication. The 15wt fork oil leaks out too easily when the bike is upside down too.

Leave it the right way up then 😉

now why didn’t I think of that? :p

Might try some on Kates Toras. They need a service.

I’ve never had the oil leak out of any of the rock shock forks I’ve had in that way and I do cycle the forks upside down alot and leave them upside down for a while too.

The oil you mention should be fine IMO, I would use it.

it leaks out of the U-turn knob on dual airs. Quite common apparently and a PITA. Just wondering if I need to grease the seals. What to use? Wedtite Teflon stuff (red) says it is suitable for forks and is pretty slippy. Finishline teflon seems a no-no a I don’t like the feel of it.

I tend to leave the forks upside down for a couple of hours to lubricate the seals. But if I leave them upside down over night there will be a puddle on the floor.

Got an answer from someone who builds off-road buggy suspension:

transmission fluid not the best as its made to transfer power not reduce friction.

5W40 engine oil would be fine.

Rockoil is pretty decent.

And get some silicone grease for the seals.

loco tuning said yesterday – rock and roll super slick grease, definitely no white lithium grease