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Onan 5500 Generator ... what oil??

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Clay L:
Note that the last of the two quarts takes a long time to fill. I fill the filter before I install it and that helps reduce the time a little. When it gets super slow I also screw the dip stick back in and turn the engine over some. That seems to drop the level enough to speed up adding the last of it.

I used 30 weight for years since I don't run it in cold weather but have switched to Mobil One 10W 30.
A number of folks posted that Onan customer service said that was a good choice.

Gary RV Roamer:
For a gas generator, most people use plain SAE 30 because they don't ever run in cold weather (below freezing) and SAE 30 is optimum for warmer weather (per the Onan Handbook). 10W30 or 15W40 is optimum if you want to cover sub-freezing temperature operation as well. The API service level should be SJ, SL, or SM.

Foto-n-T:
Go with the weight that Onan recommends and stay with a name brand oil. As afchap said, I wouldn't waste my money on synthetic oils, (snake oil in my opinion). 

I would also recommend that you add some Sea Foam to the fuel supply in the appropriate amount.  The biggest issue you are going to run into nowadays is the fact that pretty much all fuels (gasoline) have at least 10% Ethanol.  Ethanol DOES NOT age gracefully!!  Sea Foam or Stabil will give you a fighting chance of not having to replace your carb.

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