Higher octane gas does equate to lower gas mileage (it's counterintuitive but the very things that increase the octane rating generally decrease the energy content per gallon by as much as 10-15%). It sounds like you don't have any choice in the matter, but you generally don't need that high an octane rating unless you are driving around Death Valley or the Dead Sea.
O2 sensors can make a difference. The OBD I has a very wide tolerance for sensor variability, because the catalyst on all pre-96 vehicles is for carbon monoxide (CO)/ hydrocatbon (HC) only. The default mode (in the event of sensor failure) operates with a lot of air and they can be nearly non-functional without tripping the Check Engine Light (CEL). But replacing them all after 100,000 miles can tighten up the operations for air and fuel. OBD II (1996-present) is a three-way catalyst and requires absolute control of O2 in the exhaust to keep the catalyst working correctly and not burning it up.
I can't remember for sure but it seems the pre-1996 Gen 1 Intrepids usually have 2 sensors.
Tire pressure and a good pressure gauge. I always run at least 32 psig and typically 36.
AND don't forget alignment.
Seems like there are links on the site for seafoam. Here is the commercial link:
Sea Foam
Techron is a trademarked name of a Chevron gasoline additive for cleaning the fuel system. Google "Techron."
Hope this helps. Mine gets 25-26 mpg in combination. More than 32 on highway.
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