Older engines specifically should not be run on synthetic oils due to the compounds used in gaskets and seals on the engines. The chemical make up of synthetic oils is exactly that, chemical, they are 100% engineered lubricants with fully synth containing no relation to crude oil.
Older engines use natural rubber seals, cork/paper gaskets in cases and are items which do not react well to certain chemicals, so it is quite possible that Mobil 1 or other fully/semi-synthetic oils would react adversly with them.
Stick with traditional motor oil, that's what the engines were designed for and have run well until now.
Just my 2p worth. /ubbthreads/images/graemlins/beerchug.gif