Officially its 5.1 litres but if you put that much in it in one go it would go way over the max line. Stop at about 4 and 3/4s and see how far up the dipstick it is, then go for a drive and measure again. Top up as neccesary. The on board sensor can give a good indication if it works correctly.
I use Shell Helix 5w40w fully synthetic, only because my Shell garage sells it where I get my optimax.