1999 Volvo S70 always change head bolts when replacing head

The engine is actually a 5 cylinder (which is not one of the choices on the drop down menu.) It is a 2.4 ltr turbo. The head gasket was blown and the radiator and coolant reservoir filled with a slick chocolate-colored ooze. I am replacing the gasket, but was wondering if it is always necessary or recommended to also CHANGE THE BOLTS? Or is there a procedure (e.g. length measurement) to determine if existing bolts are satisfactory for reuse. ALSO, I have seen different sources conflict on recommending putting oil on the bolt threads versus putting sealant on the threads (and some oil under the head of the bolt, of course, for torquing.) What is your recommendation.

Thanks,
Joe