Gas question for my 97 ls400?

i drive a 1997 lexus ls400. it has a V8 engine and the car Manuel says to use premium (91). in my area the gas prices are at 4 dollars for the regular (87) 4.15 for 89 and 4.30 for the premium (91). i dont usually drive that far on week days or even weekends but occasionally i drive to areas that are about and hour and half away. ive stopped using 91 for a long long time. ive been using 87. i get about 280 miles until i have to refill my tank which can hold 22 gallons. i usually go to arco which has the cheapest gas in my area. recently i filled my tank at shell with 87 and for some reason i was able to pull of nearly 400 miles with it. do you guys think there is a difference in gas at gas stations and whether you do or not should i maybe use 89 for my car? and one last thing, if i have about a quarter tank of gas left which is 87 fuel. would it damage my car when i go to refuel it with 89 or 91? thanks