The absolute best easily available oil for assembly or after run is plain old Dexron automatic transmission fluid.

It has a moisture displacing quality, so it actually works its way between any water droplets and the underlying metal.

It is a mild detergent, so it cleans the gunk out.

Finally, it is highly resistant to degradation from exposure to the atmosphere, so once it';s in the engine it will last indefinitely.

A good grade air tool oil might be better, but it's harder to find. Every grocery store has ATF Dexron.

Bill.