THE General Motors model 71 diesel, 500-hr laboratory test procedure yields results that are indicative of the service performance of lubricating oils, according to the authors. However, they caution, the oil performance is depreciated somewhat beyond that encountered in reasonably heavy-duty service operations due to the severity of the 500-hr test.
The 500-hr laboratory tests and the 60,000-mile service tests are compared on the basis of bearing corrosion, ring sticking, filter clogging, engine deposits, wear, and used oil contamination.
The oils were rated in the same order by both the laboratory and the service tests. Agreement between the two tests was very good with respect to bearing corrosion. Corrosion results of both tests were also found to compare favorably with data obtained in the MacCoull corrosion tester. Similarly, good agreement between the two tests was found with respect to ring sticking, oil ring deposits, piston deposit, Purolator clogging, and used oil examination.
Differences in air port deposits were apparent in the laboratory tests, but in the field these deposits were so light that no differences could be detected. In a similar manner, differences in AC filter clogging were experienced in the field, while in the laboratory, where the filter was changed whenever the oil became dirty, no differences were found.
Cylinder liner ridging was found to be of a random nature, the only consistent observation being that liners with low mileage exhibited little or no ridging. No differences in ring wear were observed between the several oils; however, it appears that wear does not become appreciable until the liner has operated approximately 100,000 miles.