I don't have any UOA's to speak of to show viscosity drop on the M1, PP, or RedLine, but, if I'd drain a little and add some fresh oil, the noisey HLA's would go away...for a little while, but not very long. But that way, I'm having to add more oil when the RLI can make it in one OCI. It's INCREDIBLY annoying to pay that much for OTC syns and have to change them at dino intervals, or drain and add to replenish the add pack and regain vis, plus, it's more work that I have to do. This is a VERY common problem with Miatas. Like I said, one of those things that is talked about A LOT and is the one annoying thing about the car. Mazda even wrote bulletins about the noisey HLA's. The easy fix? Change the oil. Miatas tend to generate a lot of engine heat and they KILL oil vis. On the other hand, I've heard that SOME people can go 7k miles without a hitch. The "typical" Miata driver that I've seen changes the oil at 3-5k, syn or dino, due to vis breakdown and the resulting noisely HLA's. I can't remember what the 5w40HD vis ended up at (I will have to look a Terry's email to get the numbers exact) at just over 5k miles turbo'd with HARD racing and some fuel dilution. I will let you know what the numbers were.