Never heard of Diesel in the crankcase....but I can vouch for both MMO and Seafoam in the crankcase at the recommended amounts.
Note: next have used MMO as a "fast flush" as most will document, I've only used it per the bottle directions, replacing 1 qt of my 5 qts of oil, with MMO....so basically, for a 5 qt crankcase, 4 qts of motor oil (PYB, if you will) and 1 qt of MMO..
With my latest find, the 88 dodge aries, that's been sitting most of it's life (only 29k or so on the clock....) I took the plunge, when I picked it up, I noticed the dipstick was showing low on oil, so I added 1 PINT of MMO to the case to bring it to full, then took it for a spirited drive to a friends garage were I dumped the oil out (wow, was it dirty....lol, it even still had it's "Mopar" Next oil service sticker in the windshield, and it wasn't "due" yet for service...but like I said, it's been SITTING in the Florida heat for a long time) and let it drain good (gave it 15 minutes or so), popped the drain plug back in, added 1 PINT of MMO, and 3 and a half quarts of PYB...since the bottle states to use 20%, this 4 qt sump only needed 1/2 qt of MMO (so I opted for the 1 pint bottle for ease of use).
I'm now 1000 or so miles into that OCI and my dipstick is still it's golden color....
But when I drained that initial MMO+oil out, that oil had a dark black color to it
Definitely could see the oil on the dipstick..
On my other cars, 07 Kia Rio and 08 Kia Optima, I used the 1.5 oz per qt of Oil in the crankcase every OCI....and no problems. They were previous "rental"/lease cars, so "should" have been kept up on maintenance but immediately after each purchase I always used one dose of Seafoam in the crankcase and that first OCI was always nasty
Bottom line, a flush here and there won't hurt I don't think, but flushing EVERY oil change likely isn't necessary....just use quality oil (API/GF/donut-rated Oil lol).
To this date though, black soot still is pouring out of the tail pipe on start-up of that 88 aries.....it's got some nasty build up somewhere in there, cause it's leaving black "soot" on my drive way....lol.