Welcome to the Forum!
You should absolutely continue to maintain your Boxster. Boxster that are driven often (aren't stored in the winter) and are well maintained are often less likely to have problems, like an engine failure. And with 40k miles, your chances of an engine failure decrease drastically, I have only heard of two engine failure happening above 40k miles. The oil you're using if fine, it's what I use. The 0w40 and 5w40 are just for different temperature ranges.