Originally Posted by rarebear
Always do what the book says, for TS and JTS engines 10w60 !
the issue of thicker is a non issue, 10W means that the oil behaves at 10 viscosity when cold, the 40 or 60 part only counts when the oil reaches around 100 + degrees centigrade , the 60 will stay 'thicker' than 40 and thus reducing oil consumption, the thinner the oil the more it will reach the combustion chamber and will burn off. Leaving you with the bill of filling up every 1000 km or so !!
Well, this is not actually 100% correct as the basestock is what's important. The fact is that true synthetics don't burn off so easily as the molecules are of uniform size. The above is true however of petroleum or "hydrocracked" oils.
The manner in which petroleum and synthetic oils burn off is important.
As a refined product, petroleum oil molecules are of varying sizes. So, as a petroleum oil heats up, the smaller molecules begin to burn off. Deposits and sludge are left behind to coat the inside of your
engine. In addition, as smaller particles burn off, the larger, heavier molecules are all that is left to protect the engine. Unfortunately, these larger particles do not flow nearly as well and tend to blanket the components of your engine which only exacerbates the heat problem as friction builds-up.
Synthetic oils, on the other hand, because they are not purified, but rather designed within a lab for lubrication purposes, are comprised of molecules of uniform size and shape.
Even if a synthetic oil does burn a little, the remaining oil has the same chemical characteristics that it had before the burn off. There are no smaller molecules to burn-off and no heavier molecules to leave behind. Synthetics contain far fewer contaminants than petroleum oils since they are not a refined product. As a result, if oil burn-off does occur, there are few, if any, contaminants left behind to leave sludge and deposits on engine surfaces. Obviously, this leads to a cleaner burning, more fuel efficient engine.
It is also important to note that synthetics do a much better job of "cooling" engine components during operation. Because of their unique flow characteristics, engine components are likely to run 10 to 30 degrees cooler than with petroleum oils. This is important, because the hotter the components in your engine get, the more quickly they break down.