I have looked on here and got some info on the above topic. On my next service (167,000 miles), I am considering upgrading to fully synthetic engine oil. Up to this point, I have used semi svnthetic and havent had any problems at all. My question is this, has anybody on here upgraded to fully synthetic and noticed a tangible difference in enginer performance, i.e do you believe its worth the additional cost - a gallon of semi-synthetic from Nissan (£14.35), halfrauds fully synthetic (£35.99) quite a difference in these hard economic times lol.