E10 does give slightly lower mpg than E5 but only about 1.6 % , Ie about 5 fewer miles on a tankful .
This is because ethanol produces about 33% fewer calories by volume than petrol. Thus with 10% ethanol its 3.3% less 'explosive ' than 100% petrol (only available as expensive premium fuel) , but only 1.6 % less than E5 which already contains 5% ethanol. So yes it uses more fuel, but probably not enough to justify buying more expensive E5 or ethanol free alternatives , or adding additives.
I would suggest that your remembered 350 miles per tank was in the balmy days of summer. If you are only now measuring it, bear in mind its now winter. Fuel consumption can be significantly higher in winter,
Cold starts mean the engine needs a petrol rich air/fuel mixture for longer until the car warms up. The engine oil is thicker til it warms up. Cold air is more dense than warm air, so the car has to work harder to push its way through wind resistance. Tyre pressures go down in cold weather, and this can make a big difference to rolling resistance and thus fuel consumption. In winter you may doing more of your mileage in shorter journeys on a cold engine.
Winter fuel consumption increase is much discussed on the mk 4 forum because the cars have an impressive array of fuel monitors. You notice the mpg drop immediately. But the affects of winter also happen to earlier cars with conventional mechanicals.
