Jay Honeck wrote:

Using premium car gas (92 octane or higher) is completely
unnecessary, if
you've got the STC.

Our old engines were designed to run perfectly on 80 octane fuel.
87 - 89
octane regular unleaded car gas is perfect for them.

Not exactly true. Some STCs require the use of 91 octane (or better)
autogas. There's a good reason for it, too. It all comes down to the
piston's compression ratio. Some of the older engines were designed
with a higher compression ratio and required the old 91/96 octane
avgas. The lyc. O-320 (160 hp version) and O-360 are two examples.
Your pathfinder uses the 235 hp low compression version of the the
O-540. The higher hp versions (250, 260, etc...) have a higher
compression ratio and require the use of high octane fuel.

If you use lower octane fuel in an engine designed for higher octane
fuel, you run a serious risk of detonation. This will eventually
result in bad things happening, like blowing a hole in the piston or
cracking the cylinder head.
John Galban=====N4BQ (PA28-180, with a high octane autofuel STC)