If you hear no audible knocking then you are not doing damage to the engine. There is no difference in the cleansing capacities between different grades of fuels so don't buy into that argument either. Lower grades will not clog your fuel injectors.
i use regular (87 in california, the lowest grade) on all my Lexus (brand new 05 LS and 98 LS as well). i don't think there is any harmful effect (or so i hope )
'14 MB E350 White on Tan
'13 BMW X5 X35D Black on Black
'91 Lexus LS400 Black/Gray. My first Lexus (sold at 230,000 miles)
'97 Lexus SC300 Silver/Black (sold at 80,000 miles)
'05 Lexus LS430 White on Tan w/o Nav (sold at 29,000 miles)
'98 Lexus LS400 White on Tan w/ Platinum upgrades (gone at 170,000)
I also have a 93 ls400 and ever since i've had it, i've put premium (91). The manual states: "premium unleaded gasoline with a research octane no. 96 (octane rating 91) or higher. gasolines with an octane numberas low as 91 may be used if premium unleaded gasoline cannot be obtained." I'm in California and premium is 91.
Premium fuel is required for the high compression engine. The higher temp burn reduces carbon build up on the cylinder heads, valve stems, valves and exhaust ports. The way I see it the engineers that build these engines know what they are doing and I will pay the extra $10-15 a month for the premium.
Just my .02 worth