Originally Posted by
ChickenHouse
Yes, it is indeed correct to point you to the POH, but it may be misleading as well. If you do training in an aircraft which was designed i.e. for 87 octane use and now fly on 100LL, the POH is not written for that and at latest, your FI should teach you what to do with the excess lead.
And this is why it's important to read the POH that belongs to the aircraft, and not something copied off the internet, or a generic pilots notes book, or something else.
Here's how things should work in theory:
The aircraft, at some point in time, was manufactured and certified to run on a certain number of (then available) fuels. The POH should include any notes if there's anything specific for these fuels. If, after release of the POH, a new fuel becomes available, then the manufacturer should form an opinion as to whether and how that fuel can be used. If it can be used despite the original POH saying no, then the manufacturer should bring out a POH supplement. This is added to the original POH and legally becomes part of it. It should list all the consequences so the supplement will have the same structure as the original POH: Description, limitations, emergency procedures, normal procedures, weight and balance, ... Some of these sections can be empty: Switching from 87 to 100LL will not have any effect on W&B for instance.
If you have an aircraft whose original POH said "87 octane only", and you don't have any supplement that says "100LL is OK too", then flying it on 100LL is simply illegal and should not be done. Regardless of what the FI tells you.
But this is why it's so important to read the actual POH from the actual aircraft you're going to fly. That's the one which contains all the applicable supplements. A generic one off the internet somehow won't.
And now what happens in the real world: There are practices that deviate from both the POH and its supplements. Sometimes that's a good thing (*), sometimes it's based on OWTs, and sometimes they're downright dangerous. As in the example above: If your FI taught you specifically not to lean, and you take the aircraft out for a long cross-country basing your fuel consumption on book numbers, you're going to end up in a field. Or worse.
(*) As an example, EASA decided to do a wholesale certification of a large series of engines, both Lycoming and Continental, to run on mogas. They effectively circumvented the whole manufacturers POH supplement procedure and brought out their own STCs.