if its internal i'd be looking at injectors...
many years ago i had a VN crapodore that had been left sitting for a few years...
i did an oil change, left it a few weeks, went to go for a drive and oil light came on.
turned out that an injector was gummed up in the open position and then the fuel pump kept pumping fuel into the engine, which seeped past the rings and thinned all my new oil down...
scary shit when you pull the dipstick and its way high with oil and stinking of fuel!!!