one of my friends (he's 60 and him and his mrs drive around in a very obviously modded mk7 fiesta zetec s tdci, lowered all the way hispec brakes etc ) has always put some 2 stroke oil in every 6 months to a year in all his diesels and is doing it still in his fiesta. never had a problem and it must aid pump lubrication so sticking a decent amount in may well help in this situation.

also, sound advice from Alvyn...definately do that as it will constantly dilute the petrol and lesson the effect.