Yes, propane is refined from crude and therefore will track lock-step in price with your heating oil. You are also correct that BTU content of propane is lower than heating oil possibly erasing much of your savings. A quick Google to compare BTU content and comparing the efficiencies of your oil and propane furnaces can give you a ballpark on cost benefit.
It sounds like you're on the right track running the numbers to find the lowest BTU cost.
I would search the archives for discussions about crossover points, lockout, switchover, etc. That's an often discussed point.