As far as synth & dino, well, that's what I've always heard.
But then ya all got me more interested so I did a quick search and got this info ....
Mixing brands is not the best practice because the additive systems in each may be different and when mixed may not function as efficiently as desired. However, no serious problems will occur if you must add a different brand of equivalent quality between oil changes. It is very important to maintain proper oil level.
And the main question answered here....
Mineral oils are based on oil that comes from dear old Mother Earth which has been refined. Synthetic oils are entirely concocted by chemists wearing white lab coats in oil company laboratories. The only other type is semi-synthetic, sometimes called premium, which is a blend of the two. It is safe to mix the different types, but it's wiser to switch completely to a new type rather than mixing.
Despite their name, most synthetic derived motor oils (ie Mobil 1, Castrol Formula RS etc ) are actually derived from mineral oils - they are mostly Polyalphaolifins and these come from the purest part of the mineral oil refraction process, the gas. PAO oils will mix with normal mineral oils which means Joe public can add synthetic to his mineral, or mineral to his synthetic without his car engine seizing up. The most stable bases are polyol-ester (not polyester, you fool). When I say 'stable' I mean 'less likely to react adversely with other compounds.' Synthetic oil bases tend not to contain reactive carbon atoms for this reason. Reactive carbon has a tendency to combine with oxygen creating an acid. As you can imagine, in an oil, this would be A Bad Thing. So think of synthetic oils as custom-built oils. They're designed to do the job efficiently but without any of the excess baggage that can accompany mineral based oils.
(Note from Hyper.....I don't think
this is the synth oil we use)
Pure synthetic oils (polyalkyleneglycol) are the types used almost exclusively within the industrial sector in polyglycol gearbox oils for heavily loaded gearboxes. These are typically concocted by intelligent blokes in white lab coats. These chaps break apart the molecules that make up a variety of substances, like vegetable and animal oils, and then recombine the individual atoms that make up those molecules to build new, synthetic molecules. This process allows the chemists to actually "fine tune" the molecules as they build them. Clever stuff. But Polyglycols don't mix with normal mineral oils.
If you want to read more about oil, check out 'The Engine Oil Bible'
, where most of this info came from.