This is a "Studebaker thing" but if you are using modern formulations of even diesel oil, the sacrificial zinc & phosphorus has been removed (ideal, 50-50 combination in the range of 1400 ppm) which helps avoid cam wear etc on flat tappet engines. The reason is that Z&P in oil, like the TE Lead that used to be in gasoline, are poison to cat converters, and some sort of cats are found now even in most diesels. The damage is much slower but still..... There are two ways around this--use certain Racing oils, which are synthetic priced--a good example is Valvoline VR1 Racing Oil, which still have the good stuff in the right quantities (I used this product, by the way, in summertime, as the only weight available in Canada is 20W-50, although 10-40 (yeah, go figure THAT marketing decision) is also available in the US. Cost is about $6.50 a litre/quart, retail and tax in, in Canada. There are a few other oils available in the US which have the right proportion as well. The other option is to use regular oil whether detergent or non-detergent, and add a quality additive like ZDDP. (No, STP won't work) Either way you slice it, it means the oil or oil+additive will cost you in the range of $25-30
This whole schmozzle is ABSOLUTELY CRUCIAL to newly rebuilt engines for at least the first and second oil in the rebuilt engine, and 'may or may not' be necessary for continuing use afterwards.
As cheap insurance, why take a chance?