Hmmm. I use RP in my car. 20W50 though...as I'm in the tropics (HOT!!!!!).If so thats your problem, that stuff is thicker than glue. Also depending on what year Wrx you have, Royal Purple is not recommended by Subaru, and I know Lucas is not recommended by either Subaru or Royal Purple. You may have been fine with just RP but I think the Lucas killed you.
I don't drive my car often....but when I do (say 2 times for the week..but each trip is about 70-80 km)...I drive hard, and go into boost alot.
My car's at stage 1.5 (if I can call it that) and I'm using about a 1/5 of a quart every mth.
Sounds bad or good? Plus I've found that the dipstick in the suby's are a PITA sometimes. With the car on a lel surface, and having switched off after say 15 mins.....the foll happens:
1) Pull dipstick..wipe....push in....pull again. Dip's dry!!!
2) Push in and pull...dip shows oil to F mark.