My flathead 6 likes to leak the 10-40 and tends to go through it faster, I have used 20-40 or 20-50 whatever is on sale, and it works well. I have never had good luck with 10W in a Flathead, but then again, I have never owned a new, or rebuilt, Flathead. I have always been able to get them running and drive them as is. That's part of the experience I enjoy these days, driving them instead of going out into the garage and looking at them sitting on an engine stand like I used to do 35 years ago.