That advice is highly likely to lead to the engine blowing up with (probably No 3) conrod exiting through the cases in around 500 miles. The only way to preserve the engine is to strip it completely, clean out the sludge traps, clean all internals scrupulously to remove old oil residues, fit an oil filter and only then run it on modern (detergent) oils. Otherwise, the detergent washes the old residues off the engine internals, they collect in the (tiny) sludge traps and block the oil path to the big end. Hence the loud bang and wrecked engine. The 500 miles comes from the Healey brothers who made a good living in the late 1960s and the 1970s from people putting detergent oils in Square Fours and needing a new set of engine parts as a result.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.