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.
Winter tyres do work, I havnt ran them myself but know a few that have, the biggest downside is if roads are clear and temp rises above 7 degrees, they wear quicker than a chocolate fire-guard. as the rubber compound is designed to work under this temp. for those that say we get no snow, ill ask if we get none and we dont need winter tyres like some of our neighbouring countrys that have a lot more snow than us, how comes when we get a covering this country stops in its tracks, now some will be down to driver training but tyres play a huge part in this, if you need prove that tyres make a difference take your car on the beach or across a muddy field and see how far you get.