Well first it should not cost $25 for an oil test. I used that price as a MAX as I noted. Based on Rotella POA and esters you should be able to go longer then that be that is me; unless you engine burns bad.
Next, Rotella is not the oil I would choose if given other oils but leTs play the game. There are better oils for the same price.
1. So you do (2) oil and filters changes per year ($51) here. Also note you know ZERO on how the engine is running (performance wise)
2. You do one oil test for lets select and avg. $$$, so $18.5 it is.
3. So now you have $69.5 for 2-oil and filters and (1) oil test for 100 hours, 1-year
4. Next lets say, as you stated, you can go 75 hours on one oil change per oil change.
5. Using that data, you are not doing 2-changes per year but 1-oil and filter every 7 months. So after 1.5 years your cost would be $140 IF you did 2 oil tests aong with 2-oil and filters.
6. Now after the second test you do not do them all the time. You only did 2 to see how your baseline compared. Once you get a trend that is your pattern.
7. After the 1.5 years you do some basic math and that is it. If you are using a good oil, it still should be good. You do not need to test anymore since you have your baseline.
Note: oil tests is not only to see the oil but to see how the engine is running. It is more then testing the oil. How about monitoring the engine performance, as I noted above.
8. After 1.5 year it will be $51 for 18 months of oil and filters (or 2-oil and filters). Using oil testing method along with less wasted oil & filters, less time messing with oil and the same protection.
9. Using your old theory it would be $76.50 for 18-months laong with more usd oil, filters, etc.
As I noted if you are doing oil testing only for lengthening you oil duration, something is very wrong. Oil tests are priceless. You can see problems well in advance using this procedure.
Also note that if you use a top of the line synthetic oil, you can go longer with the same protection.
