If you have been using mineral oil you might have got some harmless residues / crusts in your engine.

Synthetic oils have got additives to avoid these residues and get and keep them in solution.
If you go from mineral to higher grade synthetic some residues might just come off in a bigger piece ...

This is just a DIY approach now.
I would remove both driver and passenger door cards to get to the wiring.
Then I would try and locate the feed wires for the motors and CUT them, then cross passenger switch to driver motor and vis versa.

Providing the passenger side was working, this way you can ...