I've been there previously with modern common-rail ECU-controlled vehicles and always used the 350 and a splash of cetane booster in each fill-up to be sure. In January I went with my Iveco 40.10WM camper which is an old-tech turbo diesel and did the same; my mate in his old-tech non-turbo 6-cylinder Unimog U1300L used the ordinary stuff on our first fill-up as it was cheaper, but even with half a tankful on top of Spanish diesel the truck smoked badly and he reckoned it was well down on power. He changed to 350 and it ran much better. Various overlanders we met in Ceuta were topping up tanks before crossing the border, and not just because it was cheap there.
Another friend with a Perkins-engined camper was suspicious that his engine had 'lost it's edge' after six weeks in Maroc, including not starting as well, and only began to regain it's former glory after about a year back here.
Are they imagining it? Dunno, but I'd rather be safe than sorry for the extra cost. The Iveco ran fine,no smoking and no power loss, then and since. These trucks are all quite a bit heavier than Land Rovers etc so maybe the difference is more noticable.