well i've been using royal purple for the past 6 months. it has given me great performace and has upped the gas milaege in my car. i buy it at PAW on cold water cnyn in van nuys for about SAE 5-30W 5.50 a quart. with a k&n Oil Filter
it has been running great vs other synthetics i.e. mobil/valvoline/castrol.
i recently switched from royal purple to redline this past week and wow. an even bigger diffrence. better acceleration, lower noise levels from my engine (that annoying valve chatter that we vvti engine's get), and plus since it's cold it actually warms up the car a bit quicker too. bought it at PAW at a pricey 7.99 a quart 5-30W.
due to the price i might switch back to royal purple but the redline is great stufff. i've gone from regular to synthetic...to the royal purple to redline and redline tops them all so far...next up...amsoil. let's see if it lives up to the hype