Changing Oil Formulations
We have long recommended C-rated (Compression Ignition)
diesel-engine motor oil for contemporary use in Studebaker
Engines. Modern S-rated (Spark Ignition) motor oil is formulated
for today's passenger car and light truck engines having roller
lifters and catalytic converters. Oil companies removed the zinc
compounds needed by our older engines with higher-friction,
so-called flat-top valve lifters and associated camshafts because
those compounds shorten catalytic converter life.
Diesel-engine oils retained a higher level of zinc compound, but times are changing. More stringent diesel emissions requirements and subsequent diesel technology have resulted in a new CJ-4, API (American Petroleum Institute) diesel oil formulation. This designation is in addition to, and will probably ultimately replace, the older CI-4 and CI-4 plus ratings. CJ-4 rated oil contains only a trace of the needed zinc compound for our older engines and we do not recommend it for them. Common brands of C-rated oil (either rating) include Shell Rotella-t, Mobil Delvac, Chevron Delo-400, Kendall GT-1 Diesel Motor Oil, Car Quest CQO-CQ645 Fleet Plus 15W-40W, and Castrol Tection Extra.
We are not recommending those brands as there are others just as
good as long as the rating is CI-4 or CI-4 plus. As of April 12,
2007 I've seen both CI-4 and CJ-4 formulations next to each other
on the shelves with the same brand name! Be certain to read the
information in the little API circle logo on the back of the
bottle or jug of the prefered brand. Look for oil without the new
CJ-4 rating. We will keep you informed of these changes as they
This information is from Wurning Wheels.