I don't race TC (yet) but I would advise asking people at your club what they use as different oils perform differently for different tracks/evironments. - and that's in any kind of racing (as far as I know...someone correct me if I'm wrong) It all depends on surface, corner radii, speed, weight, etc. so talk to people at your club as they are the people that know the track. Especially talk to the people with the same chassis at your track.
Don't know much about springs. Try different pre-load?