For what it's worth, I run 15W-40 diesel oil (dino) in my '83 rocco
(173 k miles), because of the higher detergent content. I considered
an engine flush, but decided against it since there was the risk of
loosening gunk around seals and ending up with oil leaks. I can't say
how much it's helped, but I have no leaks significant enough to reach
the driveway. You can also get Quaker State for older engines, which
has additives that I believe help old seals to swell up and reduce
leaks.
HTH,
H2Only