Bro, I read in this forum also sometimes ago. They use brake fliud to remove the yellow stain. But carefull with the fluid. It can damage your paint work. Gud lug
here's the link bro.hope it can help u out with ur problem.
http://www.zerotohundred.com/newforums/car-care/162736-how-to-restore-headlamps-to-former-glory.htm
Don't mind me for sounding like a nagger, but don't people ever look through the forum before starting a thread? There is already enough repeated same topic threads around. Ah, forgive me. I guess in modern days, the brains should not be used that much.
Found out a good solution for this brand called GETF1. Pretty good results as it removes insects stains and get rids of yellowish surfaces. Basically its easy as you just need to rub with force till its crystal clear.
new R34GTT owner here. i just got it 2 months ago. glad to know there's a skyline club here in malaysia. have been looking around to ask for advice where to service my car in the future.