This question has surfaced on every high performance car forum I have ever belonged to....Viper, GT-R, Z06, Hellcat and now, F type. I’ve seen every imaginable response and there’s probably more I haven’t seen. During the warranty period, most every authorized Jaguar ( or whatever car you own) dealer will very likely stay with the spec oil. I have heard stories of some dealers in very warm climates, like Phoenix, AZ, moving up to the next grade higher oil because of extreme ambient temperatures. But, your car is not a warm blooded animal, it’s a machine. The thermostat is to keep that car at a certain temperature at all times, with the possible exception of a racing environment. Running the factory fill while in warranty is the correct thing to do, in my experience. Those Jaguar engineers are really quite intelligent, don’t sell them short.