Thanks for clearing that up. The wealth of knowledge on this site never ceases to amaze me!Running any diesel engine lightly loaded results in unburned fuel in the exhaust, causing an accumulation of a gooey wet substance. This reduces the ability for the engine to make the rated power and also causes accelerated wear.
The treatment once it has happened is gradual loading of the engine up to the rated output, burning off the nastiness.
The best solution is prevention, which is to make sure the engine is regularly ran at a reasonable proportion of rated load.
This is why sizing a genset to the anticipated load is important, and why generally smaller is better.