I seem to remember that there was a problem with some of the multigrade/synthetic oils if you changed to them after running the engine on non multigrade for some time. IIRC it was caused by lead build up with the std oil. The lead was then removed by the multigrade which gave problems with oilways blocking as so much of it was dumped into the oil.
If you ran the engine on multigrade from day one, then the lead didn't build up and there wasn't a problem.