He's just referring to the shaped gaskets for the valve inspection caps. The 500/550 use O-rings, the 650 has shaped gaskets. Same function.
Replace valve cover gasket every time. Valve inspection caps can be re-used several times.
No RTV, anywhere, ever. It's just a band-aid for another problem. If you MUST use anything, get some Yamabond or 3-Bond and use SPARINGLY. Just a thin smear, not enough to squeeze into the head or oil galleys.
Unfortunately, it looks like the shop that caused your issues before is still causing issues. Try using a straight edge on the valve cover and make sure it's not warped.