Ron you are right about it.........never put modern oil on the engine when it is not overhauled......the sludge inside an old uncleaned engine will be act like grinding paste that you use for grinding the valves..
the aditives in a modern oil will clean up all the old stuff in the engine
Total rubbish
Detergents are not solvents, they do not "clean" your engine.
All they do is surround particles that are not oil.
This stops them forming "big" lumps ( we are talking microscopically big , not naked eye big ) and holds them in suspension so that the oil filter car easily remove them.
As they tend to stay in solution , they tend not to drop to the bottom of the oil tank and form the sludge we are all familiar with.
And as they are too small for the course mesh filters to remove they stay in the oil circulating and eroding plain bushes, not that there are many in a M20 engine.
Thus they totally defeat all of the filtering system on an M20 unless you have the in tank felt oil filter fitted.
And for once Wikki has it right so go read the definition of detergent oils
Now as for the oil in your M20,
Any oil made today is a lot better than any oil made in 1940.
Diesel oils are all high detergent , usually a lot higher than car engine oils.
That leaves you between a rock and a hard place.
The 20W50 would be the better of the 2 choices.
THE BEST OIL FOR ANY BSA IS THE OIL THAT HAS JUST BEEN REPLACED.
So go with what is the cheapest , buy it in the biggest container that you can get and change it a lot more regularly than BSA recommended.
