slant 6 oil weight

Ford Super Heavyduty 10W-30 meets FORD WSS-M2C171-F1
Pretty readily available. One of the better options available for a relatively high ZDDP package in a conventional oil. Plenty for stock engine, stock spring presures. etc. Probably group II base oil. No API cert, and obviously they won't recommend it for cars with cats due to the potential long term warrenty issues.
PETROLEUM QUALITY INSTITUTE OF AMERICA TEST PROGRAM-Ford Superduty 10W-30