On my car that has a flat tappet cam I like to use Mobil1 0W-40. The spec on the engine calls for 10W-30 and it has a ton of miles on it, so I figure I'm fine with a XW-40 oil. Mobil1 says that oil is designed for European vehicles so you should be able to find it pretty easily. They actually publish the information we want to know, so there's no guessing or need to put additives in your oil: 1000 ppm phosphorus and 1100 zinc, which should be enough for a stock slant 6.
https://www.mobil.com/lubricants/-/...w-us-1/files/mobil-1-product-guide-rev-31.pdf