Removing 98% of the oil is okay, as long as it's the right 98%. If I let the 2% that contains the sludge that would be clinging if I did a cold oil change, I've done a good oil change. If the 2% I leave behind is the cold sludge clinging everywhere, I could have done a much more thorough oil change. Draining it hot is the key to this one.
Yes you do need to do your oil change on time. The question is what constitutes "on time"? If you are not draining hot, and relying only on the fresh oil to get the sludge out, you're going to have to change the oil more often.
The underlying principle is this: Your engine produces a certain amount of contamination over time. The only way you have to get rid of the contamination is to drain the oil. If you're not getting enough contamination out when you change the oil, you need to drain the sludge out more thoroughly or drain the oil more often. e.g. if you make 10 units of sludge in 3,000 miles and you only drain out 9 units of sludge, then you have 11 units of sludge to get out at the next 3,000 mile oil change. If you drained out 9 units then, you'd have 12 units of sludge to get out at the next one,...... If you went 2,000 miles, you'd only have 6.6 units of sludge to get out. Does this make sense?
Thanks and have a great day,
'01 C Class, Six Speed
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