This is slightly off topic, but I remember my brother telling me, back when he worked for Caterpillar, building their big 12 cylinder diesels, that most of those engines went to oil rig platforms for generating power. They ran them for about 12,000 hours, with careful maintenance, of course, and them pushed them over the side. Hopefully, they drained the oil first. I have no idea if constant speed running is better or worse than variable for service life.