What valve cover/head orings you guys use

The orings in the valve covers and the head plug tube seals have orings, What type buna, vinton etc do you guys use? Do you install them dry or lube them with oil prior to installation?

Buna is more of a natural rubber and is NOT compatible with ester based oils (i.e. synthetic). Flourocarbon o-rings (trade name Viton) are more chemically resistant and have a much higher operating temperature (250-ish deg F vs. about 400). Assuming I still had my slant six, I would probably stick with the buna because they are well within the operating range of the engine and are way cheaper...And I usually don't bother with synthetic oil.

Lightly oiled or greased on assy is a good idea.