2 Questions/concers 89 e150

I have an 89 e-150. It has been converted to run on propane, something doesnt work with the gas side of it. Anyways, one day i was about 3 hours form home and stopped at a convinience store for some beer and food, and it wouldnt start back up again. I finally started after rolling it down a hill and pinning the throttle. And latley it has been reving up very high at times while idling. Just wondering if it is a carb problem or what. I have no clue on propane conversions at all, so i am actually looking for any tips to help it run better, changing plugs, filters, whatever it may be. I also heard as soon as your oil starts getting dark brown, to change it. ??? any help or tips i would greatly appreciate. IT also has 280 000 and im headin out ona road trip soon.