Warning: I would never use full synthetic in a Diablo motor (only semi-synthetic), full synthetic could easily leak into the spark plug chambers and out the head gaskets due to the finer molecular formula...So to help provide some insight on cam wear, in the 1990's through early 2000's oil type was a concern in Corvette circles as synthetics became more popular. For early C2 and C3 Corvettes, cam wear was reduced with high zinc oils. The 1990s C4 engine design reduced engine wear design issues and synthetics became the oil of choice due to their long term stability. The concern back in the 90s was that the synthetic oils at that time were not as effective as zinc for coating the cam surface creating more heat from added friction which translated to cam wear.This logic would definitely apply as well for the Countach.
Synthetics have improved dramatically over the last 20-30 years and it would be interesting to hear from a real engine oil expert as applied directly to the Diablo engine design. I suspect what they would say is both high zinc and synthetic are acceptable but if using a non synthetic oil, change oil frequently.
My 91 Diablo has 53000 km and has not had any issues. I have owned it since 2005 and taken really good care of it doing most of my own maintenance and the car has been bullet proof due to proper maintenance. My head explodes when I read about owners who let the car sit idle for years and expect it to be as reliable as a regularly driven fully maintained Diablo.