Just FYI, if I remember correctly, the "C" value is supposed to account for the oil's initial TBN. Basically take the oil's TBN squared and multiply it by 0.8. So if the oil has TBN of 10, you end up with "C" of 80 (10 * 10 * 0.8). That is how the math works in that Google Sheet.Formula: OCI = (C*)(ave mpg)(sump in qts)(cubic inches/Hp)

C= constant which was derived by Tooslick's opinion from reviewing UOA's; he was using a "120" for Amsoil and "80" for synthetic oils such as Mobil 1.

Now, back when TooSlick came up with this formula, US was still using high sulfur gas. Now that we have low sulfur gas (for the most part), one could argue that TBN depletes more slowly, so potentially his formula under-estimates the OCI.