draining some oil before adding the Diesel sort of defeats the purpose. The idea is to get the crankcase full enough that the crankshaft and rods dip down into the liquid and splash it around like dust in a tornado. That does a better job of scrubbing than any parts washing brush can. The thinned-out oil might find its way past a few oil seals and gaskets, but unless the built-up crud is all that's preventing a leak, there shouldn't be any long term ill effects.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!