Thicker oil + heat =fade.Alot of people are using the 5 wt though . Ive been using the Belray 5wt for years but I like the 2.5 Silkolene for a real tight shock or on a real quality build. 2.5 silkolene is pretty good for inside Showa T/C cartridge fork inner cart when seeking plush or softer. Dont ever use BelRay HVI in a fork though the shock oil reacts with oxygen and gets real sticky, A local guy was using HVI in showa T/Cs and the seals were chunking and burning up and streaking the tubes real bad and they felt so sticky you could hardly ride it, He was using the stuff for awhile too in a few different customers bikes . I laughed . They didnt believe it until they unscrewed the oil cap on the bottle and it was all sticky. Penske/CustomAxis is using the Silkolene I was just at a seminar, they really like it.