Karl; don't try run the generator with the oil cover off. It will blow oil all over the place and it is not an indication of blow-by. The crankcase is a sealed negative crankcase pressure ventilation system. Also, I have never seen one have bad blow-by, ever. Many other problems, but never blow-by. I'll explain more in depth if you wish.
RV Roamer and dcg9381; Generac generators up until recently were leakers. The 55G (probably model 9592-x) typically leaked at the aluminum oil filler tube. This is two pieces and is bolted together under the sheet metal before is screws into the engine. There's an o-ring in there and it flattens with heat over time and allows oil to seep out. I would suggest replacing it with a gasket, home made or similar. Also, if there's an air leak to the crankcase, the oil will seep out. So, check the oil filler cap for an o-ring and check the valve cover gaskets for cracking. The breather in the center is never a problem. Rarely is the engine case gasket or seal a problem, but I have replaced one or two out of the many over the years. So, check the oil filler tube o-ring, it's likely the culprit.
Hope I helped.
Benzman - RV Generator Trainer