Hi guys I've gone and added about 3/4 of a litre of Shell Helix Diesel oil (10w-40) to my 1.4L Ford Focus Petrol engine! can anyone advise me whether I need to drain the engine and start again or if I'll get away with it as a one off?

It's not the ideal oil for the car but I don't think it will do any real damage. The Focus should ideally run a 5w 30 semi synthetic but you can run 10w 40 for certain weather conditions. In winter though the 5w 30 would be better as it will get to the top of the engine quicker. The only real difference between a petrol 10w 40 and a diesel one is additives as far as I know.

Many thanks James! Think I'll avoid the oil drain then!

On your next service purchase a can of wynnes engine flush,

Pour in to your engine oil filler as described on the tin,

Run the engine at 2,000rpm for 10 minutes

Then drop out the oil,

This will flush out the system,

Eliminating any carbon or clumps caused by two grades of oil combining,

Then fill up with new oil as normal,

No hassle.

cheaper makes of oil are now selling the same stuff for petrol and diesel engines, my understanding was diesel oil was a better oil than petrol oil, but this was before synthetic oils came out. 3/4 of a litre is a canny top up, me thinks you should check it more often and as a one off I wouldn't worry that much as the way I look at it any oil is better than no oil when push comes to shove, but as I said as a one off, as Lenny says next oil change put a flush in first and nice new filter and treat her to some nice correct oil, she will last longer.

wouldnt worry about it, you put so little in, which will be immediately mixed/contaminated with old oil anyway, a flush at the next oil change is a good idea if only to put your own mind at ease.