We are talking about excessive oil consumption
of TWO engines, I think. Unlikely there is something wrong with both engines, especially the same thing, unless a design flaw or improper installation
? like a stuck oil control ring, although that is a possibility. I would add either Marvel Mystery oil to the crankcase oil or SeaFoam, whichever you can get. It's likely that would free a ring. Can't hurt to clean out the oil system and is cheap
and easy to try. But your comment on the PO about battery charging
really makes me think the engines were just babied excessively and never properly broken in, back to run them hard for an extended period, maybe several hard runs. I am of the opinion that any engine
should be properly warmed up prior to being run hard, and then properly cooled prior to shutdown as well, especially if there is a turbo involved, doubly so if the center section isn't liquid cooled. Most cylinder glazing I have ever seen was in aircraft air cooled engines that were overheated by excessive low RPM
running, aircraft engines aren't cooled very well if they aren't flying. One thing that can cause cylinder glazing is getting very light rust on your cylinders occasionally, when run of course the rust is cleaned off, but every time this occurs it is sort of like polishing the cylinders, you do not want really mirror like smooth cylinders, you want the cross hatch
"scratches" as they hold oil, a smooth polished surface does not.
On Edit, I'm surprised you didn't have excessive blow by, that is real common for un-seated rings, back to aircraft, the big signs of rings not seated is high oil temps and oil turning black very quickly as well as consumption. Blow-by will put heat into the oil and of course turn it black fast due to excessive combustion by products going into the oil from blow-by. Of course on Marine
engines you don't know the oil temp, and any Diesel
will turn oil black about the time the starter is engaged.
I bring up aircraft only as I'm real familiar with them, but an engine is an engine