Make sure not to over fill your oil. Disregard the translated YH instructions, they lost something in the translation.
I've worked with hundreds of pumps/compressors with identical sigh glass over the decades, and all of them are suppose to be max filled to center of sight glass. Any where from visible on bottom of sight glass to center of sight glass is considered 'full'. Thats how I've run my YH for many many hours now, no problem ,no oil blow-by.... Blow by is very common when over filled.
Detergent without physical surface agitation is a difficult way to remove oil.
Like others have recommended use Brake Clean, it is a solvent that will not attack or damage in any way, paint/rubber/plastic,orings, anything on our guns, and evaporates quickly leaving no residue, its good stuff.
Do not use carb clean, or any caustic strong solvent that will ruin plastic/rubber. Brake clean dissolves oil very well and quick without the damage of other solvents.
If you spray brake clean in with a strong stream using the straw that came with it trying to hit all inner surfaces, and shake it around then pour it out then immediately follow blowing it out with a long extended tube nozzle on your blow gun with compressed air at 100+psi, all before the solvent has a chance to evaporate(less than 20 seconds).You'll blow all the oil right out with maybe 1 or 2 of these brake clean wash cycles...
jmo