Oil viscosity is a topic that comes up here often and for some reason, I continue to read those posts every time I see them.
You’d think I’d have heard everything by now.
Well, I probably have heard everything but my memory isn’t as good as it once was. 
Anyway, speaking of 30 weight oil, I owned a 72 Super Beetle for 35 years. When I finally donated it to charity (I wrote about it here when I did that) the original engine was still running as well as it did when I first bought the car brand new.
As the owner’s manual required, I used 30 weight oil for the entire time I drove it. I also changed the oil every 3000 miles.
My question is, why did my Volkswagen Beetle’s engine require 30 weight instead of a multi-viscosity oil?
From what I’ve read here about oil, it seems that a 10-30 or 40 would have been a better choice.
Read the owner’s manual, and choose your oil based on it. Your owner’s manual stated to use 30W high-detergent oil. It was built for it, and got maximum protection with it. Design is important. Air-cooled V-dub engines had no oil filter, and the cylinders were lubricated by oil splash. This was a carry-over from the original design from the 30’s.
All modern engines designed today that I’m aware of are designed around multi-weight oil. Modern multi-weight oils include additive packages that are essential for proper lubrication and long life. Plus, some additives that were used for '72 cars that allowed for 30W oil have been discontinued and not replaced in modern multi-weight oils. ZDDP, a zinc based additive, was removed from all multi-weight car oils a long time ago. This additive is necessary for good wear protection on older designed camshafts and lifters. But, it is no longer necessary in modern designs. ZDDP can still be found in straight weight oils and multi-weight oils for use in Diesel engines, like Shell Rotella 15W-40 oil.
Several reasons IMHO.
One is that back in 1972 much of the oil being sold then was straight weight rather than multi-weight. Now, most or all recommendations are for multi-weight so straight weights are becoming a thing of the past.
Two is that the VW air cooled engines, like aircraft and air-cooled motorcycles, run much hotter than a liquid cooled engine and a straight weight oil seems to hold up better.
In the past aircraft and motorcycle engines have generally run on heavy straight weight oil due to the fact that they’re designed with larger clearances than a liquid cooled engine and using a bit of oil is considered normal.
My old Harleys use straight 60 weight and rather than pay through the nose for the dealer only, Harley branded oil I use Aeroshell 60 weight aviation oil.
Newer engines also have much tighter manufacturing tolerances. If you live in a cold area, starting up with straight 30 causes excessive engine wear with tody’s engines. Using straight 10 will be too thin when the engine gets really hot, and also cause excessive wear. That, and the modern additives are the many reasons why many drivers get 300,000 miles out of an engine without an overhaul.
In the 50s and 60s, such mileage was very unusual.
Piston ring tension was probably low. Pistons may have been designed to become round only when warmed up. All the clearances were probably wider. In short; the engine was designed to use 30 weight in warm weather.
No. What you have heard us say about 10W30 or 10W40 is in relation to modern cars. In their owner’s manual they will have similar advice. Your old Beetle (I had a 1970) had a much different engine than today’s cars. It needed a far different oil than today’s cars.
Note: I did not say one oil was better than the other. No I intentionally said different. Also car’s today can go far longer between oil changes without harm. Things change. Most of the changes have been for the good.
I Believe The Beetle Was The Design Of Dr. Ferdinand Porsche, Formerly An Airplane Designer.
That explains the engine and some of the unique body stampings for weight and material saving “fuselage” strength. He was told by Hitler to desgin the “People’s Car” which translates to Volkswagen.