The 0W40 has a K100 viscosity only 0.6 higher than the 0W30, making it the thinnest 40wt i have ever seen.
also its TBN is only 6.6, so dont expect it to run as long as the 0W30.
pour point and flash point are VERY similar however, to they probably use the same base stock.
the 0W40 is an API SM oil meeting BMW LL04, whereas the 0W30 "meets the engine test requirements" of SM, and is LL01. LL04 is NOT for extended drain intervals, despite the LL meaning long life". Both meet A3/B3 specs, so both should have very similar wear performance, possibly even the same additive pack (just a bit less ZDDP in the 0W40 maybe?)
closely related indeed, possibly the exact same thing, but with a bit higher VI and base viscosity to knock it into 40wt territory and keep it there over the non extended drain interval, and a bit less additives to get SM/LL01 approvals