Why do most of us change oil? For me it's because I suspect the old oil is dirty. This leads to my question. If that's the case isn't a synthetic oil rated at 20K miles going to get just as dirty in 5K miles as a conventional oil. Both are experiencing the same number of combustion cycles. Maybe there's something about oil I don't understand. Hopefully someone can help me better understand why one oil is rated for 5K miles while the other is rated at 20K. I understand there are different additive packages but to me that doesn't explain one oil getting dirty and the other not getting as dirty in the same amount of use.
Oil Differences
- Thread starter FordMan59
- Start date
It's not just "dirty" in the sense of having a lot of suspended crud, although that is part of it.
Over time, a number of things happen to oil in engines- the additives get depleted due to various chemical goings-on within the engine, the oil itself changes grade (usually gets thicker), crud gets suspended, the oil can be diluted due to excess fuel blow-by, and so on. Different fuel types and engine types cause these sorts of things to occur in different proportions and at different rates, so diesels, gasoline engines and gaseous fueled engines all have different oil requirements.
The cumulative effects of all these things adds up to a need to change out the oil periodically, in order to keep the additives fresh, the oil in grade, and remove all the suspended crud. That varies by the fuel type and the usage- short-trip gasoline engines are going to have a very different change requirement than a stationary natural gas generator, for example.
And yes, oil type/brand plays into that- oils vary in their ability to resist grade changes, oxidation, additive depletion, crud suspension ability, and so on, and some are rated for longer changes or different fuel types than others. So that's why some are rated for 5k, and others for 20k, or why some meet certain specs and not others, etc...
Over time, a number of things happen to oil in engines- the additives get depleted due to various chemical goings-on within the engine, the oil itself changes grade (usually gets thicker), crud gets suspended, the oil can be diluted due to excess fuel blow-by, and so on. Different fuel types and engine types cause these sorts of things to occur in different proportions and at different rates, so diesels, gasoline engines and gaseous fueled engines all have different oil requirements.
The cumulative effects of all these things adds up to a need to change out the oil periodically, in order to keep the additives fresh, the oil in grade, and remove all the suspended crud. That varies by the fuel type and the usage- short-trip gasoline engines are going to have a very different change requirement than a stationary natural gas generator, for example.
And yes, oil type/brand plays into that- oils vary in their ability to resist grade changes, oxidation, additive depletion, crud suspension ability, and so on, and some are rated for longer changes or different fuel types than others. So that's why some are rated for 5k, and others for 20k, or why some meet certain specs and not others, etc...
I'd say it usually thins due to shearing or fuel dilution then if its an extremely long OCI it will start thickening.
I know the oil in all the cars and gas tractors thins some more than others.
The diesel tractor doesnt thin and will start thickening as the soot loading gets high
From what I understand, it's sort of a balance game between them; they do thicken over time, but they also suffer fuel dilution. Whether a specific oil thickens or thins out over an oci is very dependent on the engine, oci and other factors- it's not something you can say in a blanket sense.
