Lotus have issued a statement saying that all of their 900 series engines - including the Sunbeam's 911 type - are able to use 98 RON super unleaded petrol without adjustment. However, bear in mind that most branded super unleaded fuel is rated at 97 RON and it is therefore advisable to use a fuel additive that contains an octane booster unless you are sure of the higher rating (NB – Shell Optimax Unleaded is rated at 98 RON, and this is suitable for everyday use without any additive). Extra-hardened valve seats can be fitted, but it is not necessary to have this work done in order to use normal ‘pump fuel’ - unless the engine is being rebuilt anyway. Another option is, of course, leaded petrol - if you can find and afford it!