slant 6 oil weight

I'm not sure why a 5 year old thread got resurrected as the OP's question was already well answered. I'm also not sure what Thumpr meant by "SAE oil" - presumably a Passenger Car Motor Oil (API S-category, like SN) rather than a Heavy Duty Engine Oil (API C-category, like CK-4). OEM slant six valve trains probably don't require the added ZDDP of a HDEO but the added protection for the tappets certainly won't hurt.

My 1965 owner's manual states that 10W-30 and 20W-40 are suitable for +32°F temperatures and I run 10W-30 in my slant six. I also have an hemi oil-spring in my engine and my oil pressure with 15W-40 is always 70 psi, which is excessive. Excessively thick oils cause the relief valve to divert flow to the sump instead of the bearings. See Engine Wear and Chrysler Oil Recommendations.

I would look for a dual-rated oil (eg, API CK-4/SN) with a Ford WSS-M2C171-F1 rating (1000-1200 ppm phosphorus) and the only one I'm aware of now is Petro-Canada Duron SHP 10W-30.