My guess is, you’ve got hardened seats, and you’ll be fine, but with as little as you’re using it, lead substitute would be cheap.
OTOH, any motor with a flat tappet cam ought to be using a ZDDP substitute these days. Manufacturers have taken much of the zinc out of motor oils (ruins catalytic convertors, and isn’t needed for roller tappet cams and OHV motors). Even lawnmowers/garden tractors ought to get an SJ oil or ZDDP substitute (the zinc is way low in newer SN oils).
