I use full
synthetic 5W-40 Rotella T in my flat tappet car.
Back to the original question. A roller will generally make more power all else being equal. But it is all in the cam design for the application. Higher opening and closing rates are possible (with cooresponding higher loads on valve train components) with rollers so if we where to compare a flat tappet at the highest practical ramps with a roller again with the highest possible rate of open and close, the roller would make as much or more power in the same power band as the flat tappet with less duration. Higher cost is a factor but overall the roller is a superior design. Don't pick the cam yourself. After your head work is done and you have that data, talk to the experts. And then pick a cam. Be sure your geometry is right on the money. Circle Track ran a good article a while back (this year) on how to check and correct this. While you are at it, the new beehive springs are showing a LOT of promise in the area of valve train control.