Same here, I should be using premium in both ML and GX but both of them are running fine on regular 87.
91 should burns cleaner do to aditives but the 87 burns faster and after measuring few tanks, there is no difference in mpg out of tank. GX has still the original exhaust and oxygen sensors, now at 259K miles, both front sensors on my ML320 went bad at around 150K miles (before switching from premium gas to regular).
2003 Lexus GX470 280K miles
1999 Mercedes ML320 222K miles
The ECU detects the octane indirectly and adjusts accordingly. Put in premium and get 100% of the published hp. Put in lower grades and you'll get somewhat less, like a handful of hp lower. No big deal one way or the other.
im pretty entertained by the number of folks who choose to ignore toyota's recs. look, filling up a tank and then not noticing anything doesn't mean everything is running in top form.
the human butt dyno is hard pressed to even discern a 25hp delta up or down. what makes you think a tank of gas is going to make you feel anything different?
it's about the long term health of your rig. if it says prem fuel only on your fuel door, run prem fuel. if you're struggling to justify the diff, why are you driving a gas guzzling rig anyways? let alone a luxury suv?
the health of my engine is perfect. plugs at 65k miles were perfect. of course i cant see what the valves look like but i do use injector/valve cleaner additive a few times per year.
a grade recommendation is typically for compression issues. most grades today have good detergents.
for me, it wasnt a choice, i just so happened to use 87 for 75k miles and then just recently noticed the gas door said "premium only". somethimes i think it s ploy to get we gas guzzler owners to buy the more expensive gas. 9.6:1 comp isnt all that high for gasoline.