Originally Posted by
gigaplex
It varies a lot depending on brand, but a crude approximation is that Xwt is rougly equal to X*10cst (400cst would be roughly 40wt, which is a decent starting point).
IIRC that's only true for the lower weight oils, with higher weight, the spreads grows larger:
25~275
30 ~ 350
37.5 ~ 450
40 ~ 500
60 ~ 800
80 ~ 1100