The owner of said website is none other than this forum's own gsxr (Dave M.). No one knows 603 heads like Dave.

Oil from head bolts? My guess is that it's the head bolts around the #2 and #3 cylinders and the oil's leaking from the joint between the crossover pipe and the intake manifold and/or the intake manifold and head. AFAIK, the head bolts don't go into oil passages... or weren't mean to. It's certainly worth the price of a crossover pipe gasket and head gasket to be sure you don't have to pull the head. Plus you get to replace the glow plugs if you really need something to do.

There's no reason to replace a head for preventive measures unless you plan to circumnavigate the globe. The progression of head cracks is very slow and the warning signs are easy for a beginner to catch.

I'm from the camp that says that you don't have to replace the chain unless it has stretched or you have everything apart. In fact my 265K mile chain would still be in the SDL if not that the darn pin wouldn't come off so I cut the chain to pull the head and let a machine shop pull the pin. It had 2-3 degrees of stretch at that mileage IIRC.

Sixto
95 S420
87 300SDL