Yeah, it seems like the stp is working. I'm kind of surprised that there wasn't more zinc showing up on the report though. I'm going to keep using it though, it's worked for years, so why change it.
I'm also surprised that the viscosity was that low or stayed in the 30 weight range. I thought that STP raised the viscosity by two grades ie. a 30 weight oil would turn into a 50 weight. The report said it was a 40 weight though so do you think there was an error made ?
Well, what ratio of STP to 10w30 engine oil? If the sump is big, the thickening effect of 1 bottle of STP might not be very big.
The second thing to consider is oil shear. How much did the 10w30 oil shear, and how much did the STP shear? Enough shearing, and an oil will go down a viscosity grade, making a 10w40 into a 10w30 and so on.
My dad told me that in the 1960s, guys used STP because they thought it was the cool thing to do. Back then, it may have made sense, due to the limited detergent content of 1960s engine oil, and because not all oils had high ZDDP.
About 4.5 quarts of oil and 1 bottle (I think it's about a pint) of stp.