As DustyLBottoms mentioned low octane fuel= more BTU, and the reason your engine will feel more powerful, in lower RPM u will feel more torque, compared to high octane.
Specially that u say it has close to 180k miles, most likely intake valves are not as clean on port side and might not let as much air/fuel in as it used to when new, so cylinders don't get filled as they used to & and overall CR becomes less than it should be in new engine. Today's fuel injection's don't meter fuel like a old carburator would, so mixture always stays the same ratio, and opposite of carbs, when ambiant temp is higher your Mpg will actually get better, and when ambiant temp lower U get worse Mpg , but more power.
But if you dyno a high compression engine with different octane fuel's , there will be noticeable difference in HP when it gets close to peak RPM, but it is allways more noticeable when you use high octane fuel in a low compression motor, U loose a lot of power, and it feels weaker.
So ( in high comp engine) u might not feel a ping with low octane fuel all the time, But in the Most efficient RPM's when engine is between MAX torque & MAX HP RANGE (2800 to 5200) it definitely is not doing the engine any favore's. ( cause it will start knocking)

 

Thanks Bom2oo2. That's a much more informative and satisfying post than "just do it 'cause LR engineers—the smartest most infallible people on the planet—say so" or "beware of the head gasket boogeyman!" Also, like I asked, your answer hits on some things to watch if one really did want to continue such an experiment: RPM and torque. I appreciate your post because while I can turn a wrench and get the fundamentals of what's going on in my engine, I definitely lack the higher theory posts like yours and a few of the others can teach me and others.

 

Just to clarify, the head gasket boogeyman is real. I saw him under my bed.

 

Haha

 

I concur.

 

My point about the LR engineers was that they already did all the tests and research on this subject. No need to reinvent the wheel as it were.

 

That wasn't really directed at you, DustyLBottoms, since you also provided other helpful information before your final appeal to just heed the gods at Solihull. And I concur with the head gasket boogeyman being real as well, but either of those as arguments in and of themselves are not nearly as compelling as the actual reason or science of why something is the way it is. I want to know my Rover better, that's why I appreciate complete answers like yours and Bom2oo2's (and others).

 

No worries. I love knowing the science as well!