Synthetic vs. Conventional Oils:
The most striking difference b/w synthetic oils and conventional or so called petroleum based oils is the homogeneity of the carbon molecule(s) or -using the jargon- alkane(s) lengths. Meaning the composition and effectivly the weight of the synthetic oil is an exact defenition of the molecular composition of that oil. So, when you buy 10W-30 oil, its been engineered to have a composition of alkane's that give the oil an exact cold viscocity of 10W oil, and an operating temp viscocity of 30W oil. In contrast to petroleum/conventional or "dino juice" based oil, the alkane lengths are variable and have a compostion more akin to a polydispersed [short chains mixed along w/ medium length chains and long chain alkanes], than the monodispersed synthetic oil.
So, the weight of the conventinal oil is more of an estimate than an exact measurement, this is what makes the conventional oil more succeptible to breakdown. Much akin to knock in a combustion cycle, when and organic molecule under extreme heat and pressure can form a radical [ an atom with one lone electron] species-very reactive and dangerous- this causes a chain reaction w/in the combustion chamber, breaking down the surrounding carbon molecules and propogarting the formation of more and more radicals eventually leading to spontaneous combustion or knock. This spontanous radical formation is also possible w/ engine oil, when exposed to high heat the possibility of radical formation increases, the radical species formed is capable of reacting w/ the metals and material w/in the motor, and the surrounding oil particles-accelarating their degridation and breakdown-leading to a chain reaction that leads to the breakdown of the oil. Which can, at the extreme, cause metal to metal contact, gasket degridation-as we know rubber and plastic harden and become brittle when exposed to high temp/burned then allowed to cool.
Synthetic oil, however, is less likely to succumb to such conditions largly b/c of its greater degree of molecular homogeneity, hence it is able to provide a more consistant barrier b/w the surfaces w/ in the metal against thermal abrasion. And, it itself is more stable hence protecting the internal surfaces-sorry for the redundancy-against radical(s) and the damage therin. One tangential point, synthetic oils b/c of their molecular composition, are somewhat "thinner" than their conventional counterparts, again b/c the alkanes w/ in the oil are engineered to given lengths, unlike the petroleum based oils which are blended together during/after the cracking of the natural stock takes place. That is why some people have complained of leaks when switching to synthtic on a motor w/ high mileage and a lifetime of petroleum based oil use. Rule of thumb, if you've been using petroleum based oil for the majority of the engines life, keep using petroleum based oils, just change the oil more often.
-Hubert
[This message has been edited by bb6 (edited 29 April 2002).]