On an old diesel like this straight 40 or 50 weight will not do any damage in the summertime that I can think of. Before multigrade oils hit the market engines like this used 40 weight straight. Probably an earlier version of the 616 four cylinder diesels did for sure. The heavier oils just seal the bores better and can reduce oil consumption to less than half.

These engines although it is hard to tell because of the seemingly endless odometer tampering or malfunctioning to get a clear picture. Of just how many miles the average example can accumulate if the oil is changed as it should be. I have a suspicion that for summer weather the engine longivity might be better with straight 40 over say a a 15-40. I still have my doubts about using any oil thinner than that is good for long life of the engine.

This is not the start of an oil thread but just some observations over many years. A diesel presents high thrust loadings. The viscosity is needed to keep metal to metal contact hopefully non existant.