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15w-40 vs 30w vs 40wt

mekanik

Senior Member
Joined
Aug 20, 2015
Messages
943
Location
Canada's Northwest
Back when 15-40 came out in the early 1980's I was sent a Cummins publication on it called: "A technical overview of oil consumption" It basically explained how carbon buildup between and below the piston rings caused liner polishing and oil consumption. 15-40 greatly reduced this carbon buildup. I don't recall the technical details.
During the same time period Detroit Diesel had literature out that claimed using high detergent oils in their engines would cause the compression rings to become stuck in the ring grooves with carbon buildup. Any Detroits I had apart that the rings were stuck in the grooves I would ask what oil they were using and it was usually 15-40 high detergent. Of course these were well worn high mileage engines.
 

John C.

Senior Member
Joined
Jun 11, 2007
Messages
12,865
Location
Northwest
Occupation
Machinery & Equipment Appraiser
There is always a variation in what is recommended and what will work. I was also sent to a Cummins school where the 15/40 rule was established. The instructor showed us pistons from rebuilds and showed the stuck and blackened rings and spaces between the rings saying that indicated straight grade oil and would void any warranty. Then again he posed a situation common on the west coast where questions were asked about how something was done. One of our group asked what would you do if you were in Alaska and only had one type of oil to use? His answer always rang true. When you have no choice, you do what has to be done.
 
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