you cant go wrong with either really mate
personally id probably go for the 5W-40......its got less of a range and therefore less viscosity additives, which is a good thing
it will also flow better in the cold than the 10W, and dont worry about it being a 40, even in a 16v
a 40 rated oil is what Porsche recommend so im pretty sure it will be fine in a 16v, in fact as far as im aware Porsche wont give there approval to any oil rated over 40, an im certain a Porsche engine gives its oil a fair bit more grief than a 16v engine does !
also it may have to be Total oil mate i get for you, as where i get it from doesnt seem to hold much stock of Elf stuff
it will be Total Quartz 9000, Adi has the 10W-50 Total in his 16v at the moment, which i sold him, but i can now get it at a nice price and hes havin some 5W-40 off me for the Williams engine he is stickin in