From what I've read in other forums, Royal Purple is the best. Mobil 1 is a synthetic based on dino oil, so it's not technically 100% synthectic. I use it in my wife's car, and figure if it's good enough for Porsche -- factory-speced in the 911 Turbo -- then it's good enough for me.
I'm going to give RP a shot in my P5 and see how expesive it gets -- plan to buy a case of it. I figure $5-6/qt is cheap insurance against future engine problems. I plan on driving this car until it dies, so I'd rather pay due diligence upfront.
Also, synthetics are the way to go if you live in a colder climate because they flow better at lower temperatures. Which is why I'll make the complete switch (brakes, power steering, tranny & engine oil) to synth in the Spring. That way I can compare how it runs throughout the Summer and Winter.
From what I've read here (and elsewhere), you can't beat the OEM filter; buy a few at a time and see if your dealer offers quantity discounts.