VW recommended 30w oil in the '50's/'60's because it was a budget car for the masses, both bearing clearances and the cooler bypass system (can't think of anything else to call it right now) is designed specifically for 30w, single weight got the job done and there was no need to spend any more when multi weights (at the time) were considerably more expensive. The name of the game was low operating costs as the Beetle was sold as an economical commuter car.
That said- millions of dollars have been spent on oil technology since then. The proper multi-weight lubricates sooner on start up and with more modern refining techniques is a much better/cleaner product. I don't think there is an automotive manufacturer today that factory fills or specifies a single weight oil for their products.
The only issue an owner of a modified VW engine with higher valve spring pressures has to watch for when shopping for a multi weight is ZDDP content, which Brad Penn, Valvoline VR1 and others have (or you can dose your own with several different brands of additives). As multi weights became cheaper VW started recommending them.
Interesting- as early as 1961 in the Beetle owner's manual VW recommends using a muliti-weight- 20w20 in temps from 32- 86°F
https://www.thesamba.com/vw/fo...e.php?pic_id=1556846
and found this little tidbit (again, 1961 Beetle owner's manual)
https://www.thesamba.com/vw/ar...61bug/08-61T1-38.jpg
@Former Member- What API rating is the oil you are running?
Ps- I agree with Michael- thinner is better. 20w-50 is known to close off the path to the cooler at highway speeds because the higher hot pressures can be too high to let the bypass open, which causes the engine to run even hotter.
Another PS- Don't get me wrong- I'm not down on single weight oil. I just think there's better products (multi-weight) to spend your money on. If I was bombing around, needed a quart for the car and all that was around was single weight I'd happily use it, but it wouldn't be my first choice for an oil change.