See, this is the kind of discourse that makes this forum useful. I honestly thought MMO was thinner than a 0 weight. My basis for this is comparing its rate of flow when topping of my wife's car with 4oz of the stuff to the rate of flow of the 0w20 I topped mine off with in roughly 20F weather; both oils were stored outside and would have been near ambient temperature. The 0w20 would have been about the consistency of a 5 weight at that temperature and the MMO was considerably thinner. We'll go with MMO being a straight 5, though, and run through a few scenarios.
That said, you're off the mark by a fair bit.
Speaking in terms of vehicles with 5qt sumps, replacing 1qt (that's 32oz, for the record) with MMO, as I stated, that is 20%. You'd end up with something resembling a 9w33, close enough to 10w30 for the purposes of this hypothetical discussion.
Consider a 4-banger with a 3.5qt sump, now; also consider that your average uninformed additive user (we're a bit above average here, remember) is buying *and using* 1qt of MMO. The engine calls for 5w30, so that's what is used, except that 1 of 3.5qt is 28.57%, which means while the "winter" rating remains at 5, the second number (you know, that all-important rating of the oil's grade at operating temperatures) drops to 22.857, close enough to a 5w20 for our purposes.
Let's further assume a newbie to BITOG who has been convinced that thinner is better. Same engine, same sump, but 0w20 this time. 1.43w15.71 isn't a grade I've ever heard of, how about you? 1w16 is what it would likely be labeled, if grading were that precise; it's too thin to be considered 0w20 anymore and nothing calls for 0w10 just yet. This is just asking for trouble.
My Corolla calls for 5w30, I prefer to run 0w20 for a number of reasons. I also like to run 16oz (13.5%) of MMO for the last 1000 miles of an OCI, which would render my 0w20 a very thin 0w20 *when new*, but oil on the last 1000 miles of an OCI is *not* new, it's already thinned, already broken down, closer to what one would consider a 15 weight at operating temperatures. So, to prevent thinning it any further, it's best to not add something that's considered a straight 5 weight to that oil, no?
The solution? Mix in a heavier oil. When I was equating MMO with 0 weight, I was mixing 16oz of it with 16oz of 5w30, for roughly a 2.5w15 (again, no grade I've ever heard of, but bear with me). The result of adding this blend to the existing oil (after ensuring it won't result in an overfill, of course) is a minimal viscosity change; an increase if anything. Given the claim that MMO is closer to a straight 5, let's run those numbers again; when we do, we end up with a 5w17.5, or a thin 5w20, something still more acceptable than we would end up with by further thinning an already broken-down oil.
I was neither defending nor decrying the use of additives here, you clearly didn't take the time to do the math yourself, and your source is dubious at best. So, what were you on about, again?
