Stirlingengineforum.com

Du pont'
Has new synthetic oils that are nonflammable and are stable up to 760f. This could give you the option of adding rings to your piston for a better gas seal.

You need to find an aircraft maintenance engineering shop, that services gas turbine/jet engines, they use a fully synthetic oil that would be about right. Another lubricant would be what's known here as Super Lub, it's Teflon based. Ian S C