sure, you can go back to conventional oil... just make sure it's not an energy conserving type.
okay, if your petcock is working properly, when the bike is parked/not running, the gas overflow you are seeing is most likely what is leftover from the fuel line at the petcock and down which is being allowed to flow from that stuck open float valve.
when you're riding and come to a stop, the engine is still producing vacuum allowing the fuel to flow to that stuck float valve allowing fuel to flow freely into the carb and out the air filter and into the crankcase.
I would think when starting the bike the idle is sort of not normal and the bike runs poorly until you're up to speed?
if you have the means, pull the spark plugs and I'm sure whichever cylinder has that stuck float valve will appear blacker than the other. That way you'll know which carb needs cleaning.
