This is from Petersen's web site. They have an STC for auto fuel use with a 160 hp engine in a Cherokee 160 which used the O-320-D2A, which is the same engine as what you have, except for prop mounting configuration. Just my opinion, but it would seem to me, that as long as you had proper flow to the carb and you were using 91 octane or higher octane fuel and were observing the other requirements of the STC, you should be OK using 91 octane and above mogas in your RV with an O-320-D1A.
Good Luck,
Mahlon
PA-28-160, -161, -180, -181
Beginning with serial 28-1761, PA-28-160, -161, -180, & -181's are approved for 91 octane mogas. The installation of this STC requires the removal of the Piper installed electric boost pump and the installation of two different electric pumps. This change in pumps was deemed necessary when this airplane failed to pass the minimum flow tests. It's a redundant system, you can run one pump or the other but the two cannot be operated simultaneously.
Other modifications include the replacement of a fuel line fitting in the fuel selector and placarding the airplane for takeoff and landing on the right tank only.
Much of the plumbing on the new pumps is done in our shop to save time when you have the kit installed. Your IA will have to tighten the fittings and mount the completed pump assembly on the firewall, reshape the line from the fuel selector to the firewall fitting, move the gascolator, change the fuel pump switch and install wiring. Placards on the fuel filler openings and on the instrument panel must also be installed. Installation should take approximately four to six hours.
PA-28's of this series with serial numbers below 28-1761 may have the STC installed if the engine installation is revised to that used on number 1761 and up, and if the line from the right tank is at least 3/8 inch diameter. Doing so however, requires a different exhaust, cowling and prop. It's totally impractical unless you need to rebuild the airplane from the firewall forward anyway.
The complete kit for serial 28-1761 and up, including pumps, fittings, placards, installation instructions and Supplemental Type Certificates sells for $1725.00.