where the oil does not load up with carbon (and it's acidic byproducts) nearly so quickly - thus the mileage between changes can be increased and provide some savings through synthetics reduced tendency to break down. In diesels, on the other hand, prolonging the period between changes, can be counter-productive, due to the diesels increased production of carbon and acid contamination. Both high quality dino oil and synthetic provide excellent lubrication initially. I prefer to use Rotella 15 - 40 in all our diesels (both company and personal) and feel it's imperative to change oil at the end of the season so the bearings are bathed in fresh oil for the storage period. I don't see the need for changing again in the spring before returning to service (it's the same new oil as it was in the fall). New oil in the crankcase really does not degrade significantly in the that period of time anymore than it would if left in the container on the shelf.