I think the main issue is wetstacking (
http://cliffordpower.com/wp/wp-content/uploads/2011/12/IS_09_[B]wet_Stacking_GENERAC.pdf), which is most known on diesels but can affect propane/NG engines too from what I have read. The main problem occurs if the generator is operating for short duration and light load and can never reach operating temperature.
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As far as efficiency, my 21kW diesel generator has an Isuzu engine, and it scales down fuel efficiency extremely well at lower loads, to the point were there is almost no penalty for running at lower loads compared to having smaller generators. This was unique to Isuzu units; the Kubota and Mitsubishi powered units do fare worse when running under light loads.