# OCI Formula

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.
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.

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.

No factor for diesel vs gas?
Which diesel engine in particular did you have in mind?

Huh. Comes up less than Audi recommend 10k oci.

Which diesel engine in particular did you have in mind?
Cummins 6.7 (deleted)

Cummins 6.7 (deleted)
deleted?

Anyway, depending on how you operate it, I'd probably use the "based on turbo/DI and hard driving factors" result - the middle one of the three. If you tow a lot or otherwise fall under "severe duty", I'd select "yes" next to the "Hard driving or racing?" field and see what you get.

Deleted means EGR and DPF were removed by the previous owner. Without all the soot getting recirculated, OCI can theoretically be extended (at least based upon soot condemnation).

I recently purchased the truck and at 2,000 miles on the oil, you can barely see it on the stick. All of my previous diesels turned the oil black pretty much as soon as you started the engine after the change.

It does, but I drive so little, I don't come anywhere close to hitting it. I believe it's scaled for 6K intervals.
The OM says 6K miles or 6 months max, whichever comes first.
*A 6K mile / 6 month (which ever comes first) using synthetic oil is a safe , easy to remember OCI - EXCEPT if you have a turbo / DI engine . Therefore my non - turbo , non DI vehicles get the 6K Mile / 6 month OCI where as my DI engine Sonata gets dropped back to 5K mile / 6 month OCI (all are mixed city / suburbs / H' way driving) .

Did I do this right??

80x30x6.4x(212/268)= 12,149.76

80 x 27 x 5.5 x (110/122) = 10 692
Owner's manual recommended OCI = 15 000 km (9000 miles)

dark grey prius needs total
Not anymore, Total oil is for gold cars now. Shell Helix Ultra would work well.

Not anymore, Total oil is for gold cars now. Shell Helix Ultra would work well.
Too bad the equivalent to that sold in the US is in a Yellow bottle now.

Not anymore, Total oil is for gold cars now. Shell Helix Ultra would work well.
That's why you buy oil in batches... It's cheaper than repainting your car

We've added it (along with few other methods) to an online calculator a while back. I still plug numbers into it sometimes, just for S&G.

What TBN method does this calculator use?

What TBN method does this calculator use?
Most oil manufacturers measure virgin oil TBN using ASTM D2896 method, so that's what you should be using here.

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