Vehicle is Acura RL 3.5 1999? Also what mileage should I expect from the same? Thanks

you can change back at anytime. you do NOT need to flush anything.
synthetic oil simply has more detergents and does not sludge up.
switching to regular will hurt nothing. you MIGHT get a little less mileage, and if you can notice the engine not idling as smooth, you need to be on TV.

Think about this.. the blended motor oils, synthetic and regular oil combined.. there's more regular than synthetic in them. those are a waste of money.

yes you can regular you change your oil every 3000 synthetic every 5000

Should flush the engine if you are going to switch back to regular oil. Mileage interval for service with regular is 3 to 4 thousand and with synthetic typically you can go 5 thousand or so.