If it was a nice new tight engine 5W30 would be fine, though it could use a little as it is very thin.
I would go for a 40 weight, ie 10W40 or 5W40 etc.
I always flush, some say not, I have been for 25+ years, my cossie has just hit 200,937 miles on the original engine, although I had rebuilt the head, the crank and shells look new, but they're not, have history since day one didn't even really need a polish, bore wear was present of course, but still doing 40+mpg on a run, co 0.97% and 137ppm at mot time, on greys this I put down a lot to the oil changes.
Prior to it drinking big time(last 1000 miles or so) it used little oil which was changed every 12,000 miles with a flush everytime, I use WYNNS, on every car.
tabetha