I would have the rear main seal replaced, in the car, by a competent shop (someone who isn't afraid to work on old cars) with a rope type seal and a Sneaky Pete (tool). I would use conventional oil. My experience is synthetic is more prone to leaking but I'm sure some will disagree. I would stay away from the diesel oil. I guess 20w50 is ok, just fortify it with ZDDP Plus so the flat tappet cam is protected. If you get the seal replaced rin 10W30. If you're in southern California the 30W Penn Grade high performance (not break in) would be good and doesn't require additives. It's expensive though.
https://www.amazon.com/gp/product/B006LB7AY6/?tag=gbody-20