Petrol

 This question comes up a bit with classic car owners and there's not just one answer. Assuming your cylinder head has hardened exhaust seats, which most do by now, this section should be fairly easy to follow. Modern fuel, like modern oil is the same but very different from the fuel (and oil) your MGB was designed to use. Modern fuel is designed for modern engines which have brains to constantly adjust engine parameters to suit, and no longer have inefficient carburettors, opting instead for direct injection which bypasses the intake valves completely. None of our ancient features like preset advance curve ignition, cast iron bathtub combustion chambers with Siamese ports, carburettors and low volumetric efficiency are considered when blending modern fuels. Fuel in the 60's was pure petroleum with a very small amount of tetraethyl lead added for octane and valve cushioning. Modern fuel is petroleum made up of many hydrocarbons with lots of additives, some of which enhance our engines and some of which don't. The high compression MGB engine was designed to run on 5 star rated fuel but would run on 2 star if needed. Fuel back then was essentially rated for compression ratios with cheap family runabouts having low (sometimes in the mid/high 6 to 1 ratio) compression for use with cheap fuel and performance cars, like our 8.8 to 1 MGBs having higher compression ratios for use with more expensive fuels. The star rating went from 2 to 5 star which was 90 octane to 100 octane. 

So, modern unleaded fuel burns completely differently in our older engines. Modern fuel has "about" the same burn temperature although it is slightly lower, but it also has less total energy content which means it produces less power for the equivalent amount of fuel. It also burns slower than the fuel our engines were designed to use and modern engines run hotter than our engines to give better emissions results. So, the slightly lower burn temp along with higher engine operating temps it's designed for, and slower burn rate means that older engines need work to make this new fuel burn properly and make anywhere near as much power as it made with old fuel. Also, most modern fuels have ethanol mixed in which does increase octane (and heat of vaporization) slightly but lowers the total energy content yet again and is an aggressive solvent which can be bad for some seals and gaskets in old cars. Ethanol is also hygroscopic which means it absorbs water vapor directly from the atmosphere and our fuel systems (and crankcases) are vented to the atmosphere (i.e. not sealed systems like modern cars). Water absorption in ethanol fuels can happen as quickly as 2 weeks from blending and remember, that fuel you just bought may have been sitting at the refinery, then in a truck and then in the tanks at the service station for longer than that when you buy it. Always buy your fuel from a busy service station if you can.

That all sounds rather ominous but don't fear, your MGB can be made to run reasonably well on modern fuel without too much trouble. Once you're at the point of tuning your car yourself you can adjust the SUs and the ignition timing to suit this new fuel. If your MG is stock and you just drive it around fairly sedately, just fill up with non-ethanol U95. If you're having issues with pinging when you labour the engine, back you're timing off slightly using the knurled wheel on your 25D distributor if you have one fitted. If you have the more common Lucas Australia 29D as fitted to most MGBs, minis, Mokes etc. from around '68 to '74, you'll have to loosen off the retaining plate and rotate the entire distributor anti clockwise a couple of degrees. If you don't want to mess with your distributor yet, you can try U98 fuel and see if that helps. I always used U98 in my stock engine MGB as it performed better and got slightly better fuel economy and it does keep your engine cleaner and running better. I still use U98 in my stage 2 engine mainly because, like the standard 18GG, it does perform better and I run my ignition advance on the very limit and U98 doesn't ping when I really get into the accelerator pedal. Incidentally, ethanol fuels have a better resistance to pinging but have the other issues mentioned before so like everything I mention here on my blog, educate yourself and decide what you want to do.