RP is overkill...a conventional 5w20 is all the car needs to perform nicely. if you want synthetic go for it but you don't have to spend RP money to get it. Castrol syntec usually runs around $30 with 5 qrts and a filter at most autoparts stores. (the filter is usually a higher end one too like k&n or bosch)
Royal Purple’s advanced additive technologies, such as Synerlec, enable its products to outperform leading synthetic and conventional lubricants. These unique chemistries strengthen the base oil for improved performance. They also create an ionic bond that adheres to metal parts for continuous protection.
Synthetic oil has a different flow pattern, it may still be 5w-30 or 5w-whatever, but it is still thinner than conventional oil, Therefor on an older car, you may find leaks where there were none before, also your valve train might be a bit louder due to the thinner fluid, their is nothing wrong with using synthetic oil on an older car, it is not going to set it on fire, it's just something you can consider when thinking of changing to Synthetic oil, Most "synthetic" oils are actually a blend, Look for things like Castrol syntec, or Mobil 1 to find full synthetic written on the bottle.
I used to use Castrol High mileage Synthetic Blend, but recently I switched back to my old faithful Pennzoil High Mileage conventional, and my valve train is a bit quieter, and the sound of the engine is a bit smoother, And a pin hole drip I used to have, is not there anymore, So these are all just things to consider.
No performance gain that I can speak of, but some people swear more expensive oil gets them more horsepower.. But if I spent 9$ on a quart of oil, I would want to feel more power as well.
no an answer to why you'd run it hard are things like racing it, a lot of driving...things of that nature lol. take the new 5.0's for instance....ford recommends 5w20 for gas mileage and 5w50 for track use in the same motor