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If it's not really about tuning or wiring. Then it belongs in here.
As the tittle says...
I have done it many times, warm engine drop oil, refill with diesel to full capacity, run it for like 30 seconds and drop it.
Refill with cheap oil, run it up for 1minute and drop the oil.
Refill with premium oil and new filter.
This drops heaps of crud and your oil stays gold for much longer most often done it to engines that have blown a HG and have oil and coolant milkshake.
Question is can this short exposure to diesel bring any harm to seals or anything?
Is there a better alternative with an equal or better result?
That's an expensive oil change.
As long as there isn't any load on the engine, then the short running with the diesel shouldn't* cause an issue, but it will wash the oil film off fairly quickly.
You can do 14 laps of a race track using petrol as an oil replacement in a Ford Crown Vic.
*Check with your engine builder.
I always learn something from HP Academy -- Kids don't try this at home.
this has been a common practice for years in the 80-90 -2k before dpf was introduced, I have never heard of or seen any bad side effects, im not sure about newer diesel engines tho,
It's a horrendous thing to do to an engine, even if a common practice by some.
Other than the washing off of the oil from surfaces that rely on the film to protect hitgh load areas like camshaft lobes and lifters/followers/rockers, the biggest issue is the very thing you seem to be doing the practice to achieve.
That is the loosening of deposits, what you call crud, into the crankcase oil which in turn gets drawn into the pickup - with the potential of partially blocking it, through the pump - with the potential of scouring the softer body and sticking the bypass, through to the filter which 'should' catch the crud before getting into the oilways but if it blocks enough crap will open the bypass to the oilways. That is well and good, but if there were deposits built up over time in the oil-ways and those get loosened off, there's only one way for them to go - into the bearings and other oil feeds, which may result in debris in the bearings and potentially blockages of fine oilways.
If you were to flush the oilways to clear the mayonaise, I honestly don't see any possible advantage over using a cheap oil WITH FILTER CHANGES for every one of the oil flushes. If you're really concerned with coolant contamination, adding some ethanol to absorb water "might" help, but I'd just do an extra oil/filter change.
TBH, I'm rather more concerned why you feel regular oil and filter changes aren't good enough - unless you're using the wrong oil for your application, or running it too hot/cold and/or having some other issue, I can't see why you are still getting the 'crud' in the oil.