I was recommended not to use fully synth in my Disco. Reasoning went that the engines work best with the type of oils they had when they were designed, just plain old mineral, or semi-synth if you must. The newer synthetic oils are effectively more 'slippy' (smaller molecular structure) and can leak out through smaller tolerances than normal oils ... which means they can easily get burnt up when/if they bypass piston rings, for instance, but 'cos it's a small amount of oil they don't make as much smoke so don't get noticed ...
Having said that, it sounds almost plausible enough to be an urban legend to me ... but I still run mine on mineral ...
OTOH ... mate filled his up one morning, went for a drive, all OK. Next day went for a drive, engine felt lumpy after an hour, checked and no oil .. Oil cooler pipe connector stub had cracked and it had all leaked out within an hour ... might be worth checking the oil pipes etc, 'specially near the oil cooler ...
