Suppose you take a premium conventional oil Delvac 1300, Delo 400, Chevron Supreme XW30 etc change it at 3-4k/ intervals. (daily driver, highway/street driving) Then on the other hand running Mobil 1, Amsoil, GC, RP, and RL on the other hand at 5-5k intervals. (or 8-10k depending on UOA). My million dollar question is at what mileage interval can synthetics maintain OPTIMUM engine cleanliness?? (on a non bypass filtration system that is) My first going assumption is that by a certain mileage the oil will be holding its capacity of chemical contaminants and microscopic abrasives. Second, going assumption is that as the sythetic ages so will the likelihood of deposit formations. The dilemna I have been pondering has been 5k on synthetic may not be as overkill as thought of.