The Gas Turbines were synthetic, the Diesels were Mil-L-9250, conventional dino used by the military since I joined in the 70s. My coach has set about 11 month a year and last year we again only put about 300 miles on it. Oil quality has changed so much in the last 70 years that the old norms are no longer a fact. My first car they recommended 1000 mile oil changes and SB rated Oil 30 weight in the summer and 10W weight in the winter, My 2019 Jeep requires SN rated oil semi synthetic, quite a difference, about 12 revisions in the Oil Specification since the 1955 Plymouth and the 2019 Jeep.