I think it's equivalent to SAE20 and is probable fine when new, but after forty or fifty years it degrades into a foul smelling watery mess. But I speak from experience with using a heavier grade in the Armstrongs, it made a pretty drastic difference to my Morris Minor traveller and a big improvement in the vans. Makes sense if you are upgrading the front dampers but I fitted some new old stock ones to the front of Harvey and he rides well.