My rule of thumb on the octane issue is thus:
If you have KE-Jetronic and no knock sensor, do NOT use anything but
premium on the highway. Midgrade is OK around town, but you really
need the knock protection on the interstate as the engine cannot retard
the ignition timing if it knocks. Burnt valves is the usual result.
On later models with LH-Jetronic and knock sensors, remember that
retarding the timing will reduce both power and milage -- you will have
to run both, keep accurate records, and decide whether the lower price
is actually a savings for you own particular circumstances. I know
that on several of my older cars, including my Mom's old Fairmont
station wagon and the work van (with 300,000 miles on it!), the
accumulation of crud on the piston crowns raised the compression ratio
enough that regular rattled pretty badly. Premium cured the spark
knock, and the increase in power and milage more than paid the
difference.
It is entirely possible to have the milage drop so badly from retarded
timing that it's more expensive to run regular than premium. You also
risk damage to the catalytic convertor/oxygen sensor running late
timing on acceleration all the time.
Peter
