5w30 and 10w40 are the grades listed by Vauxhall for any petrol Omegas.
I would use 10w40 semi-synthetic in all of them. We don't have cold enough weather for the lower viscosity rating to be of any use, and it is more likely to leak and make the engine noisier on startup. It costs more too.
Brand doesn't matter as long as it meets the spec; buy whatever best fits your availability/price combination. An oil and filter change should be less than £20. Do consider the recommended service interval to be a maximum, and vary it according to your use: low mileage/only short trip cars are candidates for shorter mileage intervals, whereas high mileage cruisers will justify longer. My annual mileage used to be sub 9k, mixed between local commuting and several long trips, and that was how I did oil changes as part of the basic plugs/filters/is anything about to fall off checkover.