Two schools of thought, on my older 2001 GP, I used regular (Organic) oil with an additive (Prolong) that I will swear by. I put it in my older Ford Explorer too and you can hear the engine smooth out as it gets into circulation, no joke. I got that GP with 30k and now have 198k on her, even though she's just sitting there now, it starts right up and dip stick is clean as a whistle.
On my new (older, lol) 2000 GTP, because of the more aggressive nature I plan to drive her and the higher heat extreme's (6,000 rpm) I put 100% Synthetic in it and won't consider anything else. Synthetic just doesn't break down as easy as organic, but again, if you change it when you should, then it shouldn't matter. With normal driving you can extend intervals between changes so the cost is offset over the long run (3k organic -vs- 5k or more synthetic between changes). The synthetics like Mobile 1 have detergents in them as well, that'll keep things cleaner. Suggestions not to use Synthetics in older engines stems from the older synthetics running thin and leaking where there was no problem (leaks) with using regular. Personally, like now, I've never encountered that problem, not saying it's a good/bad idea at all.
Just my $.02... ultimately, it's a matter of choice
