These engines are a 1940s design, happy to work on dinosaur oil... you need some zinc, just like the oils in the '60s had. A hi-tech modern oil is wasted on these old engines.
For all my cars i use branded diesel engine oils.. they are much cheaper, widely available in 5 litre and 18-20 litre sizes, lots of detergent and no 'catalyst compliance'.
Search for API CF4/SG 20W50 oils, i see a variety in the UK - 65 UKP for 20 Litres