OK, guys, I actually did a LOT of research on this subject when I bought my minor about 6 weeks ago, and also in relations to my MG Maestro last year when the valves went. This may go on a bit, but will hopefully answer any questions you have...
The first thing I noticed, is that the majority of mechanics don't really know what they're talking about, and it's easy to get a different story from different people. I'm saying that what I know is absolutely right, but with the number of people I've asked about it, I think it's fairly accurate.
My Maestro's valves went in July last year, just in time for the MOT. I took it to my usual garage who said it was because I'd been running it on unleaded. He charged £700 to do the valves (which I've since found was vastly overpriced). While he was doing it, I checked my engine and ECU numbers with Rover, who confirmed what I already knew - that the engine was suitable for unleaded. I told my garage this, and he said that although it had hardened valve seats, then timing couldn't be retarded which was what caused the problem (this is also wrong, as it is fitted with an ECU with the timing already retarded for 95RON unleaded). The solution, he said, was to put a cylinder in the fuel line containing pellets. He put it in for £25 saying that would solve my problem and sent me on my way. Unconvinced, I contacted the MG Owners Club. Firstly, the pellets are supposed to add lead (or something) to the petrol and have no effect on the engine timing (so my mechanic obviously didn't know what he was talking about) and we'd already established that I had hardened valve seats anyway. More importantly, these products have been tested by the FVHBC and they have no effect at all. Didn't matter in my case, of course, as it was an unleaded engine anyway, and they don't do any harm. Now what I found out when enquiring about an unleaded conversion for the Minor (which my wife tells me is now known as Naomi!), is that if the bottom of the engine is weak and the head is overhauled or replaced, then this will increase the wear on the pistons, rings, etc. That's something to think about when getting an unleaded conversion on a Minor. Sure enough, 12 months later, the piston rings went on the Maestro and 2 weeks ago I got a reconditioned engine.
When I picked it up from the engine place, we had a chat about unleaded on older cars and he said that in the example of the Maestro, it was OK, the timing was right and the valve seats were hardened, but it was still an engine originally designed for leaded and has higher compression as a result (a Rover O-series). He said that the engine would last longer and perform better if run on 98RON Super Plus, although 95RON wouldn't really do any harm. He also confirmed that the pellets did no good at all.
So, that's the case with the Maestro, and I would assume that a Minor would be similar in terms of the octane rating - although the timing can be retarded, you're better off using a higher octane and leaving the timing alone. Another thing I learnt, is that if a car's been driving for a long time on leaded, a 'lead memory' will have built up, effectively cussioning the valve seats for a while even if it's run on unleaded. I would imagine that's the case for those using pellets or standard UL on an unconverted car. The FVHBC has approved a few additives which work. I use Castrol Valvemaster in the Minor because it's easily available in Halfrauds and is recommended by the MGOC as well. Apparently the reason you should mix even approved additives, is that each additive sticks well to the valve seats and provides lubrication, but apparently they don't stick to each other very well, so if you keep changing, it doesn't get a chance to build up sufficiently. This is the same with LRP - the manufacturers won't say what's in it so you don't know if you're mixing or not. Also, they probably contain less additive than they should, and all the petrol companies refused to submit any for FVHBC testing, which I think demonstrates their own confidence in their products!! It's disappearing from forecourts now anyway, and good ridance to it!
The advice I was given for Naomi was to use an approved additive which would apparently work almost as well as lead. The car's only done 50k so apparently it's an unnecessary risk for me to fit a new head, and I'm better off waiting until (if) the engine eventually dies of old age, then getting it fully reconditioned and converted for unleaded. This seems to be working fine. I'm using standard valvemaster with standard unleaded with no pinking and with the engine timing unchanged, although next fill-up I'm going to try with super plus and see if there's any difference. Then, if there's an improvement, I'll probably use standard UL with valvemast plus - then Tony Blair gets less tax off me than if I was running super plus with standard valvemaster!
I hope this answers some questions. If anyone knows something I'm missing or have got wrong, please let me know! Incidentally, did you know you can get Valvemaster direct from Castrol (sorry - can't remember the number, but try surfing the web or calling Castrol) for 6 bottles for the price of 5 with free postage or 12 for 10, if you buy in bulk.
One other tip - keep the bottle in a plastic bag in hot weather, or let some air out of it or something cuz mine expanded in that hot weather in August and leaked over the carpet!!!
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