slant 6 oil weight

I've upgraded to a high-lift, short-duration in my slant six and have been using 340 valve springs since 1987. I've always run 10W-30 except for the time I tried 15W-40. Any modern CK-4 heavy duty oil with the Ford WSS-M2C171-F1 approval should have enough ZDDP to protect your valve train. I would stay away from racing oils because they have less detergents than HDEOs, which means you'll need to change your oil a lot more often to minimize sludge formation.

Unless your oil runs really hot, you'll get more flow to your bearings with a thinner oil (like 10W-30) than a thicker oil like (10W-40). Chrysler spec'ed 20W-40 oils for summer operation so I would go with a 15W-40 if your low temperatures rarely fall below +32°F. See Chrysler Oil Recommendations and Engine Wear.