Answering your question, one thing about Jaguar engines, they tend to be bomb-proof and do incredible mileages. My XJ6 had 155,000mile on the clock and still used NO OIL between changes.
There is nothing wrong (in this case of engines) following the advice in the handbook. If you live in a very cold climate, there could be a case for using a lighter mineral/synthetic blend than suggested. But only of known-good quality of manufacture. When cold, engines are much thirstier on petrol/gasoline, and a lighter oil would help if this concern you.