The point I'm trying to make is that the optimum time for changing oil ought to be related to a number of factors, of which distance travelled is probably one of the least important in most cases. Here is my selection in rough order of importance:
- Number of cold starts (more condensation in a cold engine)
- Ambient temperature (how long before warm enough to stop serious condensation)
- Effectiveness of crank case scavenging (more of that anon)
- State of wear of the engine (piston blow-by multiplies the problem)
- Accuracy of carburation during warm-up period (extra gook produced)
- Distance travelled (well, lets get that one out of the way)
If you were clever (or anal) enough, you could probably come up with a really clever formula incorporating all those factors. However, I would give 1, 2, and 3 equal top weighting. Items 1 to 3 have to be taken together since a given number of "cold" starts in the Dakar in summer is not the same as an equal number conducted in Fargo in January. The effect in either case will be modified by how much gas gets past the pistons. What we are really after is the severity and duration of the initial condensation period. All other things being equal, that will give you how much condensate will be produced and I would suggest that more than anything else determines when the oil should be dumped.