Don't mix synthetic and mineral oils. It will clog up your system.
Best bet is to just keep using whatever has been used all this time. Start mixing and matching oils and you may end up having to clean out the oil tank and oiling system.
Maybe someone else will chime in here, but there are some synthetic oils you can get at Academy sports center, possibly bass pro, and other boating stores, and they might be able to be mixed or used to replace the bombardier synthetic.
I have had the oil mixed and know how much of a mess it is to clean up.
When I first got my xp, the guy at the shop told me that the mineral oil was the right oil, so I ran a gallon of it through the ski, and then my oiling system clogged up. I had to drain and clean it all out and reprime it with synthetic and start over. Luckily I noticed a reduction in oil consuption and started to premix or I may have lost that engine.
From then on, i use only bombardier synthetic, despite it being VERY expensive and a 2 hour drive to the shop to get it.