Why do you think the engine needs a forty grade oil?
Is it consuming oil, or is it leaking?
Is oil pressure low when fullly warmed up?
At the mileage you have on your car, if I wanted to use a forty, I'd be looking at something like Pennzoil Conventional, Quaker State Conventional or Nextgen.
You live in a very moderate climate, so a dino forty would be fine, if the engine really needs to move up a grade.
If all is well on a thirty, just use a thirty.