You need to change the oil because it breaks down and is no longer a lubricant.To do it on a 1990 KTM im not sure as i havnt done one before. www.mx-tech.com has some general fork rebuild instructions in the downloads section.This can be used a guide as well as a Wp manual.You will need some kind of wp/ktm manual to get the air gap and compression settings-or a local KTM dealer should give you help on the settings.