Yes - I think 10W40 is the recommended viscosity for all petrol-engined UK based 155s.
20W50 is a touch thicker but as you say, it shouldn't be a problem in the summer. I'd put 10W40 in it again for winter.
I use Selenia 20K (nee "Formula Alfa Romeo) - it's a semi-synthetic 10W40. My old 33 P4 ran on it from day one (apart from a brief affair with Shell Helix when I couldn't find some once) and the motor didn't burn any oil, even with 80,000 miles up, so I'm guessing the engine wear was almosy non-existent.