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I run a 1990 Mercedes 300E. Normally run it on 98 Octane but through to save cost would try and downgrade that to 91. It ran Ok but did pink a bit under pressure, so decided to go back to 98 when tank half full to get back to good running and better performance. Now however it seems to run more roughly than before. Is this just the engine adjusting again to the 98/91 mix.
Appreciate any assistance on that matter thanks.
Murray

If there is 91 octane fuel left in the tank then this can be expected. Check again when the tank needs refuelling and fresh 98 octane fuel is in the tank. If the problem persists then you could have a faulty spark plug/injector or worse and this will need to be checked by a mechanic to determine what the cause is.