I have a 01 Corolla with 285k miles, I have always used an API SL 20w50 Conventional oil and a 3k mile interval. When I began to browse through this forum I decided to start using an API CI4/SL rated 15w40 oil because I learned that a thicker oil isn't always the best option.
My car is using 1 qt of oil every 1k miles and it has a 4qt oil pan, so at the end of the interval I have added more than 3qts of make-up oil. I'm only getting 25 mpg because of the way I drive. My guess with this level of oil consumption and this fuel mileage is that I shouldn't extend my OCI too much, but I don't know if even in this "harsh" conditions a 3k OCI is an overkill. My next oil change will be with Motorcraft HDEO 15w40 CJ4 Rated BTW.
My car is using 1 qt of oil every 1k miles and it has a 4qt oil pan, so at the end of the interval I have added more than 3qts of make-up oil. I'm only getting 25 mpg because of the way I drive. My guess with this level of oil consumption and this fuel mileage is that I shouldn't extend my OCI too much, but I don't know if even in this "harsh" conditions a 3k OCI is an overkill. My next oil change will be with Motorcraft HDEO 15w40 CJ4 Rated BTW.