the silly slotted heads by hex head SS bolts on our old Volvo (Jhonston pump) and had no trouble in about 5 years use
I was always concerned abut breaking them, of the slots getting chewed up

Unlike Brian, we used Tefgel. No problems.
Our pump was readily visible every time we checked the oil or looked at the engine, so screws backing out was lower risk than in a Yanmar