If the engine was on wvo at one time. You might have had some polymerization of the oil at one time leaving stuff behinf in the engine.

I might run a container of miracle mystery oil to loosen things up for some time before the next change. Wvo sitting long term changes to a form of glue especially if air can get to it. no air just slows down the process. Remember some of the old oil would have remained in the oil heat exchanger as well.