I am on my second gallon through a GRP with an O.S. plug. No problems. The only problem I had is that the motor wouldn't start with the GRP plug that was in it. My other buddy's GRP plug wouldn't even glow fresh out of the motor.
There are 3 of us running this setup with no issues.

If you read the "Ask Mike Truhe" forum on Neo-buggy.net he even tells you that its ok.