I know that 2k oil changes are unnecassary, but my question is would 2k OCI with Castrol or penzoil conventional 5w20 offer similar protection to a synthetic at 5k OCI? I do not want to go past 5k miles while under warranty. My driving consists of some around town with a daily higway commute of 10 miles each way. The reason I am leaning towards staying wiyh conventional is the excellent UOA I have seen on this board with 5w20. I figure 2k OCI'S will also give me excellent engine cleanliness. By the way this will be on a 2003 F-250 with the 5.4 v-8. Any input would be greatly appreciated!