I personally would not use a flush. I would perform the flush by doing frequent and hot oil changes.

If you insist on flushing before putting in the synthetic, there is a product called Motor Flush. I would add the quart to a hot engine and run a few minutes before pulling the drain plug and filter for an overnight, or at least a one hour drain. By hot I mean full operating temperature. It takes typically about 17 miles to reach full operating temperature. I'm not talking about the coolant temp being at 85C. This happens in a couple of miles. The oil temperature trails the coolant temperature by quite a long interval.

I would not recommend filling the crankcase with diesel and starting the engine. You could either pour some diesel fuel through a hot engine without starting it, or use a couple of quarts of diesel with the rest engine oil.

Good luck,