you need to pressure test coolant system to rule out coolant leak in too engine block,and take out spark plugs to see if any are super clean from coolant leaking in to cylinder.good way to check...also check for condensation from exhaust which never goes away.
i know you said car runs well,but does coolant temp look normal.
And if it is head gasket,oil under raidiator cap should be more like a creamy colour.
correct me if im wrong.
i have an english skyline engine manual on cd in PDF.Covers engine and engine components on rb20e to rb26dett.If anyone is intrested email me at skondoch@hotmail.com $12 will cover cd and postage anywhere in australia.manual has 491 pages.