If you read up about oil properties and specification you find that a A5/B5 is the same as A3/B3 with properties of an A1/B1 oil, which means it has got some low friction additives to reduce fuel consumption. This is a UK or EU requirement for car, engine and oil manufacturers. Also A5/B5 will allow 18000 mile oil change intervals where A3/B3/B4 would be changed around 8000 - 10000 miles.
The thinner your oil the easier the engine will run at cold temperatures.
The lubrication and protection will not be worse with "thicker" oils like 10W 40. Most sensitive people will probably not rev a cold engine high anyway.
Volvo also specify that it should be an ACEA SM which contains some camshaft protective ingredients to reduce oil residue baking on engine parts like camshafts over an ACEA SL...
If you are not after some - to me - dubious claims that the oil reduces your fuel bills massively you can use more readily available and cheaper A3/B3/B4 as long as it is fully synthetic. There are semi synthetic A3/B3/B4 oils out on the shelves.
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