Air Conditioning

Just been to Lanzarote and had my 1st experience of driving on the wrong side of the road, coped OK but I've got a question regarding Petrol Pumps.

I only filled up once after leaving the airport and luckily it was a pump with an attendant or I think I would have been in trouble.  From what I could make out he pressed a couple of buttons on the pump, after it had spoken to him in Spanish, and set the amount of Euros for the pump to deliver.

Is this right?  

What if you think the tank can take 10 euros worth and it only takes 8, do you lose 2 euros? 

What if you just want to pull the trigger until it's full like in the UK?

Goint to Malage and Fuengirola in a couple of weeks and want to hire a car again but the petrol pump thingy is playing on my mind.  I can imagine being in the middle of the forecourt with an uncontrollable hose, if you know what I mean

Some pumps have the preset amounts available if you want, but I think it's only to make it easier to fill to the exact amount you want, if you know how much you need that is. I'm pretty sure that all those pumps can also be operated manually by squeezing the trigger until you stop, as usual. If you do pre-select, but for more than you have room for, the pump will automatically stop when full, and you will only be charged for what's actually gone in anyway. What's harder (I find) is stations which insist (usually at night) on pre-payment, which again is a nuisance if you have no idea how much your fill-up is going to cost.

Now, are there any petrol (or diesel) heads out there who can tell me what the difference is between the different flavours of diesel available, and which is best to use and why?