This is my understanding:
E.g. 5W30 refers to the viscousity range of 5-30. It actually means 5 Winter 30.
At cold temperatures the oil won't thicken up more than straight-5 weight oil, and when hot, the oil won;t thin out more than straight-30 weight oil. The same can be said for the other multi-viscosity oils just using the other numbers instead.
In a place like Singapore, where there is no winter, the front number does not really matter. Because we do not face wide range of climatic temperature change.
And to my understanding, it is ok to mix viscosity whether it is 30 or 40 etc, as long as it is synthetic. And became a norm if I am not wrong so that military vehicles can mix the different viscosities without worrying malfunction.
Just that a 30 vs 50 viscosity mix will land you a viscosity of about 40+/-. What you face is leas optimal performance because you may experience a heavier or lighter acceleration. For example, a high capacity M3 needs to use a higher viscosity so the thickness protects the engine. For a 2-L 320i, having the same mix will cause the car to have slower acceleration and your car feels heavy.
What you must not do is to mix "synthetic" with "standard" / "non-synthetic" engine oil.
But do verify with a professional/mechanic.
Cheers.
