We were told by both Mobil and Detroit Diesel engineers that the private label Detroit Diesel synthetic lube oils were made by Mobil and were virtually identical to the respective Delvac 1 products. Delvac 1 was originally offered in 5W-40 and, as I recall, 5W-50, although the Mobil website currently lists only one product, 5W-40. The straight weight synthetic lube oil is Mobilguard SHC 40. Our service department follows Detroit Diesel's recommendation of the straight weight Mobilguard SHC for Series 92 engines. Personally, I prefer the multi-vis Delvac 1, particularly for its better performance and engine protection in cold start situations. That said, either is a premium product and offers superior performance to conventional mineral base oils. By the way, our friends in motor racing use synthetic oils exclusively, be it Nascar, IRL, CART, or F1. In addition to better lubrication performance under stressful conditions, synthetics consistently dyno with small, but consistent horsepower advantages. We expect that fuel economy would show similar improvement.
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