I was curious also about switching the 500cst out to 40wt. I keep this little chart on my phone -
These are the AE oils, They have a CST rating on the bottle as well.
30wt = 350cst
32.5wt = 388cst
35wt = 425cst
37.5wt = 463cst
40wt = 500cst
42.5wt = 538cst
45wt = 575cst
47.5wt = 613cst
50wt = 650cst
55wt = 725cst
60wt = 800cst