[Closed] Fork oil does the viscosity really matter that much?

A serious question! I have a pair of Rockshox Dart 3 forks that a bit "shocking" in their performance. Sticktion is horrendous and the preload knob no longer wants to adjust without a pair of pliers for persuasion.

It can not be worth sending them for a service, that money would be better off being put towards a new pair. So, that leaves either landfill, garage rafters OR having a go myself. I downloaded the manual off the SRAM site and it looks fairly simple from a nut and bolt point of view. What worries me is the different viscosity if oil for the RHS and LHS leg. I looked on the CRC website but they dont seem to have 5W and 15W. Can anyone suggest a place to get the correct viscosity?