If the engine went for a few or many years without any oil change, and sat around for another 6 months not running, and THEN you change the oil to a synthetic, you may experience all the crud that has dried up and turned hard being washed down into the oil sump, and then gumming up the suction screen in pieces that were too large to pass the oil pump suction screen and get caught by the filter, this then could cause a "low oil pressure" situation as the pump starved for oil. But this is NOT the fault of synthetic oil, other than it is finally cleaning the crud.
It is kind of like a fuel tank with a bunch of junk, having a fuel system cleaner added that moves the junk into the fuel filter, and so a fuel filter that was OK now experiences being clogged, only a short time after adding fuel system cleaner. You should not blame the cleaner.
I have used synthetic oils for 20 years in my personal vehicles, I have converted literally thousands of pieces of equipment to synthetic oils, I have had friends convert their personal vehicles over to synthetic. Never a single failure only improved power, fuel economy, lower heat and less failure of alternators, starters, a/c compressors and water pumps. The only time I dont recommend synthetics is if there is an existing oil leak, synthetic will leak more.
The theory of hurting an engine by making a switch to synthetic was fabricated by the petroleum companies.
used minerall oil before and changed to full synthetic meens, that the additieves from the full synthetic oil will wash up all (deposits) inside the engine and this parts may be make problems to the engine, but this is normaly known!
dont use full synthetic oil, 10/40 or 15/40 is ok
Mazda 5w30
GM 5w30
isuzu 15w40