[mwebb]

using 10w40IF AND I REPEAT IF it is approved by BMW for your application is the way to go

but
more important than the viscosity values are the BMW oil specifications which are applicable for that car/engine .
BMW LL01 afaik
so a off the shelf 10w40 semi synthetic is NOT acceptable and
there are many if not all oils that meet BMW LL01 that are not 10w40
ALL OF THEM ARE FULL SYNTHETIC and meet the
ACEA A3 B3 - A3 B4 specification as well as the BMW LL01 specification

i would contact BMW direct
OR
look in the BITOG forum in the European section to clarify exactly what the minimum oil specification for your car really is and never use an oil that does not meet that specification .

and

mortals who are not lubrication engineers should never "blend oils"
unless they are salad oils for salads only
unless

unless you enjoy pain and internal engine work on older BMW engines

Originally Posted by 320touring

So the iS is overdue an oil change...

The book oil for the car is 10w40 semi synth-local supermarket had some castrol going cheap,so I now have enough 10w40 for 3 changes I may purchase more..

Question is this-could I 'blend' in some 5w30 to make it a bit thinner?

Just thinking it may be better being thinner in the cold weather we have..