Hi,
You need to stay with at least 91 octane fuel for your Lexus.
Lower octane fuel can cause problems such a pre-combustion and pinging. This can cause damage in the long run to the engine- ranging from melting the tops of the pistons, to valve damage. Contrary to what the gasoline companies advertise, engines never need to change octane. If a new engine is happy with 87 and at 40,000 miles starting pinging, that means is time for some maintenance, not higher octane. Many european cars and bikes need higher octane because of their better fuels. The US has the "worst" fuel in the world. Germany starts the fuel grades at 96 octane, the their engines are designed to need it. Now, many foreign manufacturers "de-tune" their engines to run on our 87 octane gas. If your engine needed 89 or 92 octane new, then that is what you must run. If it didn't need 92 new, and does now, look for a problem.
In my opinion, even in light of difference in fuel cost between regular and premium unleaded, it is not worth the cost to the damage inflicted slowly on the engine to change to the lower grade.
Automotive Repair shop manager
20+ years as a Automotive Repair shop manager and technician