I'm new to that stuff as well. I have done some researching on it and it seems Royal Purple and Amsoil (with the little research I have done) are on about the same level. I was in this exact same boat a couple days ago trying to decide which brand to go with and finally settled on Royal Purple personally. I still need to order it though lol. Realistically speaking, unless you are going to be using the oil in a performance setting like racing or something, they are probably all on about the same level. I know nothing about Lucas so I can't speak to that, but I have looked into Royal Purple, Amsoil, and Mobil 1 and they are about the same unless you start getting into the molecular difference in molecule bonding and retaining levels. At that point one would be better though I have not delved that deeply into the science that is gear oil to know one way or the other... The way I was looking at it for my use, I can get Mobil 1 from Autozone for 18.99 a quart for 75w140. Through Amazon, Jegs and up until 2 days ago 4 Wheel parts, they all had Royal Purple for $17.99 a quart. Taking into account local pricing being higher than online it figured out to be not much more to just get the Royal Purple. In my opinion of what I have looked into for both oils, Royal Purple seems to be better between the 2, hence my decision to just use that. Also though, a lot of shows my buddy has shown me on spike that do a lot of performance stuff for offroad or drag cars, they only use Royal Purple.
What I would suggest, look at it this way. Unless you are doing something along the lines of a performance setup/build or whatnot, you wouldn't need the best of the best. It would be almost a waste of money. At the same time, you don't want junk either. If you are looking for something that is locally available, I would go with the cheaper of Amsoil, Royal Purple or Mobil 1. Like Jeeperz Creeperz said, if you are changing the fluid a lot, you would be fine with a lower quality fluid. Any of those choices would be fine for the application mentioned, just a matter of how much you want to pay for it at this point... Best of luck with it all.
Paint