More of a general comment than based on what I've read here, be darn sure you're checking the oil correctly. All 3 of my 2800's (the only Onan I'm really familiar with) have required checking the level with the dipstick plug unscrewed and sitting on the edge of the hole. This makes a significant difference in getting the oil level correct. I actually wrote the correct method on mine in silver Sharpie. Check your manual. Be sure to allow enough drain-back time.
I agree with changing back to dino, and using a straight 30W if you don't have an unusual climate. I've always used Castrol in air cooled engines, from lawn mowers to VW Bugs.
The oil has to be going somewhere. If you were burning that much oil, I'd really expect smoke and the spark plug to foul pretty quickly. Assuming that it's sitting level, a leak should show up as at least wetness on the bottom of the pan very quickly. Almost has to be the crankcase vent system, and I'm not familiar with yours.
Best of luck with it.
Jim
