i think you should use the oil that lexus recommends.it's not a big issue as far as i'm concerned and it could help you avoid problems in the future. i'm sure the oil they use is good stuff is they put it in all of their cars.
in terms of why different places charge diffierentsly for the same service, the answer is relatively simple. because they can. in most dealers, the service department is a business unit much ,like the sales department is a business unit. they are independent. why do dealerships charge different for the same car?
with respect to following the book..i would read the manual and compare it to the dealers maintenance handbook. odds are the dealer's handbook has more maintenance scheduled than the manual. for example, my dealer suggests a 15,000 check up that included changing he transmission and differential fluid. the manual only suggets this for special conditions driving. i'm going to wait till 30,000. the differece: $120 vs. $390.