Guo is correct in that the BMI chassis requires different rear arm spacing than the standard chassis.
And with regards to the shock oil thing... that is simply what worked best in the rubber car!
I started out w/ 50wt w/ 3 hole pistons in the car and as we went thicker in oil the cars got more manageable.
In foam I tried 60wt oil all the way around as well and while it may have made the car a bit easier to drive off of center I don't really feel like it made the car any faster overall.