The oil restrictor subject is very debated on FA and mostly unresolved. Every freaking post in Advanced is like a meeting of the minds. So if they haven't concluded either way, I would have no problems going either direction. I just installed a GT2871R with restrictor on an S13, but haven't had a chance to get on it yet.

Possible wild-assed guesses:
- blockage inside the block going to your turbo oil feed?
- thinning the oil due to running super rich?
- water from somewhere?
- 15W50 is too thick for GT series?