Great question. IMO, the SD40
& 50 are only light-duty SDs, not really meant for cruising, given their built-in obsolescence (although you can now upgrade them with a real steel
roller-bearing instead of their silly copper thrust-washer, but the kit's $500/side).
I'm afraid I can't shed any light, but I'm also interested. We have a pair of 25yo 4JH2 engines with SD31s, each with 9,000 hours. They still run OK, & I've replaced most of the bearings & seals
in the SD31s, but at some point, I'm going to need to upgrade things as well.