I really like TSC's Traveller "Fleet Diesel" 15W-40 oil.
I was using straight 30W detergent, with a bunch of Lucas additive, in my 8N because it has low oil pressure when hot. You boys suggested that I try 20W-50 because you thought it might hold its viscosity better at high temperatures than the 30W with Lucas did. But I bought the 15W-40 at TSC instead because I couldn't find any 20W-50 there at the time. The 15W-40 definitely hangs in there better at high temps than the 30W and Lucas did. Next time I go to TSC I'm going to see if they have any Traveller Fleet Diesel 20W-50 because maybe that would work even better.