Regular oil tends to break down
more than synthetic oil at high temps.
The main difference between the two is that the (so-called) synthetic oil has a much narrower distribution of hydrocarbon chain lengths.
here's an example to show what I mean by this - two sets of 10 numbers, both of which which average 5. Let's assume that the number represents the length (and therefore the molecular weight) of the hydrocarbon chains in oil.
Code:
5 4 5 5 4 5 6 5 5 6 : average = [B]5[/B]
6 3 5 7 4 3 4 7 6 5 : average = [B]5[/B]
So, if we think of these two sets of numbers as two types of oil, the first set would be analogous to synthetic oil, and the second set is analogous to standard oil. Both have the same average 'weight', but the standard oil has more variation of oil 'weights.'
The second key point is this - all other things being equal, when heated, smaller, lighter liquid molecules are more easily turned into vapor than are heavier molecules. (i.e. - less energy is needed to 'kick them out' of the liquid.) Conversely, the heavier the molecule, the
more energy needed to 'kick it out.'
This means that, as your engine runs, and the oil heats up, the lighter molecules tend to 'evaporate' first.
To apply this effect to our 'oil' number sets, if we remove all numbers (molecules) less than three, and half of the 4's from the number sets, we end up with:
Code:
5 4 5 5 [COLOR="White"]4[/COLOR] 5 6 5 5 6 : average = [B]5.111[/B]
6 [COLOR="White"]3[/COLOR] 5 7 [COLOR="White"]4[/COLOR] [COLOR="White"]3[/COLOR] 4 7 6 5 : average = [B]5.71[/B]
Since the lighter components of standard oil tend to get boiled away preferentially, what is left behind is, on average, the heavier molecules, and the oil is therefore 'thicker' than it was originally. This, of course, will lead to greater cold engine wear.
With 'synthetic' oil, since there is a much lower number of 'light' and 'heavy' hydrocarbon chains in the mix, you don't get nearly as much 'thickening' of the oil over time. Sure, some of the 'synthetic' molecules get boiled away, but, since the molecules are much more likely to be nearly the same length, the oil viscosity doesn't change nearly as much.
