Gee... rough crowd. Synthetic Oils have been around for 35 years and most of the bugs have been worked out so we have to put problems found into a date range for them to mean anything.
1980 is when the Mobil-1 variety came out, 1990 is when the market opened to other brands. So let's remember Mobil held the patents on synthetics until fairly recently, when those patents expired their competitors formulations kept 'evolving' as 'real world testing' of the trade secret oil base & additives forced some versions to just quietly 'go away' while they trumped up the 'improved' versions.
That said - way back and half a forever ago in 1996, I'd used Mobil-1 for 30k miles and then tried Castrol Syntec in my 1994 Z-28's LT-1 engine and got oil seeping through EVERY gasket on the short block, and as soon as I saw it happening, I changed it back to Mobil-1 and the seepage stopped.
Best I can tell, Syntec then was similar to the earlier version of Mobil synthetic they sold between 1974 and 1980, a 'PAO' (polyalphaolefin) oil base that can be either an oil or modified into a plastics like polyethylene - and nothing sticks to polyethylene - so it seems that it will run off metal parts in hours instead of days plus was alive enough to creep past gaskets.
PAO oils are still blended into many standard oils marketed as synthetic blends but not more than 30% by weight to help stop their tendencies of run-off, seepage and other problems that were discovered seemingly by trial and error.