About 18yrs ago, I used to work for a Jaguar dealership. I remember the only oil used in those engines(inline 6cyl & V12) was 20W50, all year 'round, regardless of temperature. If I recall correctly, this was a specification that came directly from the manufacturer. Doesn't this seem a bit odd by todays standards? What would compell Jaguar to recommend such a heavy oil across the board?