My owner’s manual recommends 5W-30 motor oil, and the engine has a 5 quart capacity.  From reading other postings, I understand the MMO can be used to replace motor oil at a maximum ratio of 20% -25%.  I further understand that MMO can change motor oil by one grade.  Since MMO has an approximate viscosity of a 5W oil and assuming a 20% replacement ratio (1 quart), do I need to use 4 quarts of 10W-30 oil, or can I use 4 quarts of 5W-30 oil for this application?  The question is when do you increase oil viscosity to compensate for MMO?

Since the recommended oil is a 5W30, use 1 quart of Marvel Mystery Oil and 4 quarts of your regular 5W30 oil.  The final viscocity will be still be in range.

Rich, I appreciate your prompt response...thank you.  What if the recommended viscosity is 10W-30 with the same 5 quart capacity; what viscosity oil would you recommend for this example-15W-40 or 10W-40?

Most recent oil studies have shown less wear and damage when using a lighter weight oil than a heavier weight.  For a recommended 10W30, I would still use the 10W30 and not go to a higher oil weight,