Just a fact on the viscosity issue.
In order for a lubricant to be classified in any SAE grade (10W-30, 10W-40, etc.) it has to meet certain guidelines with regard to viscosity ("thickness").
For example, it makes no difference whether it's 10W-40 petroleum or 10W-40 synthetic, at -25 degrees centigrade (-13F) and 100 degrees centigrade (212 degrees F) the oil has to maintain a standardized viscosity or it can't be rated a 10W-40.
Craig
P.S You can go to the site below and download the Oil Bible, has more info on oil, and how it's tested and rated than you'll probably ever want to know. Just click on bonus report three.
http://www.trustmymechanic.com/wethankyou1.html