Which oil?

[Gazza]

Hello, my Crossland X 2018's manual says that for group one countries (which includes the UK) down -25C they recommend SAE 0W-30 or SAE 0W40, SAE 5W-30 or SAE 5W-40. For petrol engines, needs to be dexos1 Gen2.

The first service by a Vauxhall approved dealership used 5W-30, the next three used 0W-20. Two were the same Vauxhall place, the last was an independent garage and may just have copied the last garage.

When searching online places like Euro parts, Halfords and the oil manufacturers they tend to say 0W-30 now. One said 0W-20 was wrong for my model and others say it's OK. 

Why would the main dealer go for the lower numbers? Is this level of lubrication better for the wet belt with less warm up time or something like that? Is the thinner oil less harsh on the rubber or something?

Also, is a runnier oil at lower temperatures likely to have disadvantages? Could the 0W-20 oil get past piston rings or cause external engine leaks where a thicker oil wouldn't?

What are the advantages and disadvantages of the other oils for this vehicle? In theory it's OK to put 5W-40 in this engine? There must be a reason the main dealer went low.

Any ideas? Thanks.

[Vauxhall Owners Club]

The trouble is its so hard to tell what the weather is going to be in the UK so therefore choosing temperature related oil weights is a pure guessing game.
Hotter countries used to change between 'weights' (viscosity) from summer to winter and vice versa.
Nowadays the oil weight is structed to be variable between temperature ranges

More importantly than anything is the suitability (ACEA, SAE ratings, etc) which are crucial to engine wear and protection. Ensure it is correct and weight is not as important