I used Permatex High Temp thread sealant on all my senders. It appears to be electrically conductive so it won't interfere with the ground connection. It is available at any auto parts store.
I also use Valvoline VR1 10W30 in my FE.
Oil temp runs about 185° F and is consistent in all seasons. I don't have an
oil cooler hooked up. The
oil temp sender is in the top of the wide area of the pan. 20W50 seems a bit thick, but it is best to check with your engine builder.