synthetic extended interval, where's the dirt

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Jan 28, 2003
I've learned a lot reading threads here this year. Faced with the comment that synthetic oil is of no value in a garden tractor because of dirt, I did a search and and found no directly related thread, so I read what I could and typed this up. Do you agree, disagree? Can you expand, what would you delete? UOA on my garden tractor with Mobil 1. As far as dirt is concerned, we will rule out leaking gaskets that can introduce dirt into the oil sump. After that the oil sump is a closed container where oil can protect engine internals for many years. You see this in a transaxle where oil is seldom if ever changed. The problem with an engine is the combustion of gas in the cylinders, and the need to draw in air from the outside. There are two main problems, dirt in the air, and soot from burnt gasses. Most of this is pushed out the exaust valve, but some will always get past the rings and enter the oil. If you look at my oil analysis you'll see that I have only a small amount of silicon in the oil. At 75 hours I changed this oil not because it was dirty, but because after 75 hours I expect it to be dirty, and without analysis we would have no way of knowing. So why wasn't it dirty? Engines rely on the ring pack to produce compression, and to keep all this crap out of the oil. The rings work better if they are clean and move freely in the ring groves. The rings flex up and down on each stroke, and, excluding engines with a ring pin, will rotate around the cylinder. As engine run time increases, the ring pack will become packed with carbon, soot, and coked oil. For oil to remain less dirty the ring pack needs to remain less dirty, allowing it to do what it was designed to do. Good hot oil can clean out the ring pack, that't why and engine that is run long hours will last longer. Most of the carbon and soot builds up durring cold startup, and coking happens when you shut off a hot engine. Coking occurs when oil is subject to hot metal, and there is no cool fresh oil moved in to replace it. An engine running synthetic oil will be cleaner through out. In a clean engine the ring pack will be more effective, and the oil will remove heat faster without sludge which can act as an insluation. Since synthetic oil has a wider temperature range the ring pack will be more effectvie sooner after startup, and will be less impacted by the heat left behind at shut down. Yes, synthetic oil does get dirty, but it is effective at slowing that rate. If you can filter the oil below 1 micron both dino and synthetic oil can be run indefinately, but the dino won't fare as well due to shearing, cokeing, and cold start flow. Either way, some oil will have to be changed out from the sump to remove the dirt, and replenish the additive package, and maintain TBN. [ November 02, 2003, 01:05 PM: Message edited by: greencrew ]
True that is what the air filter is for, but that is a constant, it does not change when deciding which oil to use. The issue with a garden tractor is that is run in a dusty dirty environment, making synthetic oil a waste of money because the oil gets dirty faster.
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