[Chris Burkard]

I think contamination is not a big deal. If you filled both tanks at the same station with the same hose, if one is contaminated, both are going to be. If you have a generator, it likely pulls from the Starboard tank. I think leaving the cross over open is not a big deal. If you really want to see how much each engine is drawing, then close it and keep track of how much fuel you put in each tank and if there is a difference, a trend will appear.

Good Luck!