Well, Mark, if you want to mix a Group IV or V oil with a conventional, you've only got two choices where you don't have to order the oil and have it shipped to you: Mobil and Castrol (German variety). Since we can't seem to get a good handle on exactly how GC is formulated, I'm not prepared at this point to recommend mixing GC with Castrol GTX. SO, given your objective, I would go with the Mobil 1, but I would reverse the ratio, 3qts M1 10W30 and 2qts Mobil Drive Clean. You certainly will NOT hurt your engine going the route originally recommended to you, but it is not the mix I would use. I try to make recommendations based on what I would be willing to do in MY engine, because if I don't believe in it what business do I have recommending that you do it? Again, that being said, it is YOUR engine after all, and the mix you first proposed will, barring unexpected internal problems, give your engine decent protection.