Hoo-boy, where to begin.....
Octane retards combustion. Regular 87 octane gas actually has more energy released than premium 93 octane. However, it will detonate sooner. Before computers were involved, the rule of thumb was to run an octane equivelent to your compression: if you were in a car with an 8.5:1 compression ration, you could probably do well on 85 octane, but a 9.1:1 probably needed at least 91 octane. If you still had pinging, signs of early detonation, you move up to a higher octane.
Now that we have computers built into cars, they watch for signs of predetonation, and automatically adjust the timing to prevent knock. So, lots of people thing that it's okay to run whatever fuel they want.
Although the computer can prevent knock, that doesn't mean that the engine is a happy environment then. When the timing is being adjusted to prevent knock because you're running a lower octane than it is set up for, then your mileage goes to pot, and you do end up with a little more unburned fuel in the exhaust (which leads to a quicker deterioration of cats and O2 sensors - not catastrophic, but sooner than it would have been....) and additional soot particles through the exhaust system, too.
The change from 3.9 to 4.0 included changes to the valve specs (aside from the computer changes and loss of a distributor); however, they were too close of a tolerence. It's (usually) fine, as long as all is well. Thing is, if you're the type of person that saves a buck by running mid-grade (or regular) gas in a vehicle designed for premium, you're probably someone who doesn't change your oil as often as you should, either... try to get an extra thousand miles or so out of it before you change it.... That only aggrevates the situation, leading to oil-crud caking onto the pick-up tube in the bottom of the oil pan, etc. etc. And, age (mileage) factors in here... a brand new Rover treated that way won't be as suceptible to problems, as would one that's been around the block "several times" (most heart attack patients are middle-aged or older, right?). So, with time, the valves don't continue to slide as freely as they should....
Not every single person that runs regular grade gas and goes 10k between oil changes is going to need a valve job, and, some very well kept Rovers that have only premium gas and frequent oil changes still need valve jobs. However, usually the behavior matches the diagnosis: running lower-grade fuels and stretching oil changes often leads to having to do a valve job. It's not instantaneous. Don't think "Oh, I've done if for 25k, there's no problem...." This is the kind of problem that takes awhile to build to the point that something has to be done. 50k, 75k, 100k... you may be fine 'til then, maybe not. Keep doing it for over 100k and you either have a vehicle that the tolerances were a bit different so it can handle it, or, it's just a matter of time.
The earlier 3.9 with the distributor seems to tolerate mid-grade in an acceptable fashion over longer periods of time. I still have the wife run premium in hers, though. But with a 4.0, I wouldn't do it, not if I had it to do again. It took over 80k for me to learn the lesson the hard way. It was the post-mortem when things started clicking, the "Bob's your uncle!" moment.....
Hey, it's your vehicle. Run diesel in it, if you want.... don't ever change the oil, if you don't want to..... just make sure that I am not the person to whom you sell your Rover in the future......
IMHO, FWIW, YMMV......
-L
