I have just had a rebuilt exchange engine for my 1981 camper - Type CU 2
Litre Air-cooled and it is using HD20 oil while it is running in for the
first 500 miles (max. 50 miles per hour). The engine rebuilder
recommends HD30 after the running in period.

I have read some of the archives which explain the merits of synthetic
oil however I would welcome advise from people who have used both types
of oil and their recommendations.