To synth or not to synth, that is the question.
I think Royaiii makes the most important point. Synthetic oils are detergent oils by nature. Therefore, if you have an engine that was not cared for in the past and thus has developed a severe case of "gunk" on the inside, the synthetic oil will clean and release this crap from the internal parts into the flow. This creates an outstanding opportunity for clogging of the filter and/or the oil channels within the engine, which then leads to a really bad F'n dream. I would be hesitant to change to synth in an older car unless I am VERY VERY confident about the cleanliness of the internals and I know the oil change & maintenance history of the car very well.
As for Synth vs. Dino oil. Who really knows? Unless your racing your 900hp machine through the streets of Monte Carlo, I don't think it makes a damn bit of difference.