Nice new seals like that could probably handle SAE 60 (although your performance would suffer). Nissan calls for special 7.5W30 in the Skyline GT-R, but nobody follows that either. If you want to retain your good mileage, get a decent synthetic and be done with it. The dealer just wants you to depend on
him. They make money on the oil and a "nominal fee" for changing it for you. Put that cash to better use and pick up some
Amsoil 0W30. If you want good mileage, this is the ultimate. You can go for one of the slightly cheaper synthetics, as well. The point is, don't let those nazis push you around. You're too smart for that.