"tian yu
True . . . but . . .
Yes true . . . auto fuel can be used for flight as stated by the STC, however . . . what has the POH/AFM has to say about this?"
Nothing. The STCs are "supplemental" and issued after the original type certificate and POH/AFM, ergo the nomenclature. When you pay for an STC, it is issued for a particular airframe registration and serial number and a particular engine serial number. It is not a "blanket" STC.
All auto fuel STCs require some modification of the airframe and engine. Some mods are as simple as attaching a placard near each fuel filler cap and an Adel clamp with the STC number on a pushrod tube. STCs for larger engines such as those on DC-3s require the engines to be opened up and the cam timing to be reset.
Addenda to the POH/AFM are also issued.
Not all airplanes that use the same basic engine can be STCed. For example, the Citabria 7KCAB and many Cessna 172s use the venerable Lycoming O-320. The Cessna can be STCed. The Citabria can't because its fuel system is more prone to vapor lock, which is the major concern when using auto fuel.
When the STCs first appeared back in the early 80s, leaded auto fuel was still available. Lead keeps the valve stems lubricated and helps cushion the impact on the valve seats when the valves close. The STCs require unleaded auto fuel to be mixed with 100LL avgas in a 3:1 ratio. Alternatively, unmixed unleaded auto fuel may be used if every fourth tank of fuel is 100% 100LL avgas.
I have operated large fleets of STCed trainers on auto fuel for years and years with no problems related to its use besides having to filter it very well. But you do need to know how to do it properly. I know of operators who made photocopies of our STCs and doctored them to show their own registration and serial numbers. But the certificates themselves are only two sheets of paper. The entire STC package is about 12 pages thick for a Cessna 150 or 152.
Note that there are all sorts of STCs issued for almost every popular aircraft in service. Look at all those Cessnas with Robertson STOL conversions or floats. Cessna never approved those conversions. If you take your brand new Cessna and have floats and a STOL conversion installed, your Cessna warranty is immediately void. You will not find a POH/AFM published by Cessna that will tell you how to operate your STOL floatplane in any library or aviation bookshop.
The two leading suppliers of auto fuel STCs have issued more than 74,000 STCs since 1982.