I am just sampling, not dumping. The latest oil sample went out yesterday to OAI and should have results to post back on Friday or Monday. Yes I have recently switched. Previously the truck had Cenpeco S3 15W40 in it (Cenpeco S3 Data Sheet). The reason I switched was purely because Amsoil sponsored Diesel Power Challenge 2019 and provided me with 10gal free of any engine oil. I chose 15W40 Heavy Duty Diesel Marine since I've run it in the past as a child (all my dad used in his pickups/HD equipment) and the results were good. It also was all that was used in my 2005 duramax by the previous owner. At 342k when the motor was torn down (replaced due to bent connecting rod) all the bearings looked absolutely brand new and held clearance within manufacturer's spec for new. The engine in this 2002 is a little tired (some blow-by that I don't like, lower oil pressure than I'd like which is 20psi hot/40 psi cruising down the freeway hot and towing) but it hasn't shown me any signs it needs to be replaced yet. I am planning on running the remaining 7.5 gallons of Amsoil 15W40 HD Diesel Marine through this engine but will only change once this forum or the testing indicates a need.


Before this year I've always done 5k drain intervals on all my diesel pickups. Yes, I understand this was wasting money as the oil did not warrant a replacement but it was what I've done since I was a child and kept up with it regularly. My other truck gets an oil change every 8 sled hooks or 1000mi but it also makes 1300hp+ and money isn't an issue there. I eventually plan on going back to Cenpeco S3 oil once this stock of Amsoil is depleted. They were also gracious enough to provide me with 10gal of Torque Drive ATF (my favorite synthetic ATF for an Allison) that will be going in the transmission once it is built this winter.


The reason for upgrading the cooler is to manage oil temps while towing heavy. Duramax trucks are quite known to run hot oil temps when towing quite heavy (~15k+). The new 2020 oil cooler is a cheap investment as it is roughly $250 for all the required parts to install. I do have the truck tuned but only run on a 75hp tow tune. Coolant temps stay in check (under 200F) as well as trans temps (under 200F) when towing in 90F-95F ambient conditions grossing 24k lbs.