For the first time ever (8 years) I just had the oil filler plug blow off my 2000 FLHRI. I had just completed an oil change and after adding 4 qts of oil, she indicated a 1/2 qt low (I have a oil cooler installed), so I added 1/2 qt. Now it showed just a tad over full; I didn't give it much thought... Out for a check ride and when I got on her, she blew the oil dip stick plug about 15' and flung oil everywhere! When I got home, I removed 4oz. of oil to get it to the full mark on the dip stick. I removed the air cleaner to check the vent tube and I could blow into the crankcase with the oil filler plug removed, but could not suck any air out! Is this normal? Is there a check valve or PVC valve in the crankcase vent? Just to be safe, I removed a bit more oil to the 3/4 mark. I've always keep her topped off to the full mark, but I have been reading that even Harley recommends only filling to the 1/2 or 3/4 mark... Comments?