I agree with Norm. You can use 87 PON in a M103, but you might have to modify your driving habits a bit. For example, with my 5-sp. I need to ease back into the throttle on a short shift to avoid a second of transient detonation, and since I only drive my Merc in the winter it never sees high summer temps that increase the tendency to detonate. An auto trans will likely have less tendency to detonate since torque converter slippage will keep from loading the engine up at low revs like you can do with a manual.

I know of no reason why a lower octane fuel will shorten the life of the catalyst, absent significant detonation, which will probably hole a piston or valve before it harms the catalyst.

As a general rule, lower octane fuels have one to three percent more energy than premium grades, so if anything, fuel economy should be better, but tough to measure since the difference is so small.

Duke