For what? car , truck, tractor? Petrol Diesel? What grade? How old is the engine?
Is it for just a service? Most motor factors will sell you 10w40 in 20ltr drum for about 50euro semi synthetic. If thats what your after...!
Can't help with the 'bargain' side of things but I would advise caution in skimping on engine oil.
Only go with a brand you know or at least that has been recommended. Regular oil changes are the single most important contributor to good engine condition - poor quality oil can undo a lot of good.
My old Ford Fiesta goes thorugh quite a bit of oil. Tesco do 2L for €7.99. If you have to top-up regularly I think it doesn't matter about using the best product; it's not there long enough to make a difference.
The Tesco oil is sold as semi synthetic? What doe this mean and is it o.k. to put this stuff in my car?
I kinda thought that this stuff was for lawnmowers etc?
No, semi-synthetic is not just for lawnmowers. Fully synthetic generally offers better mechanical, chemical, lubricant properties than 'natural' oil. Semi-synthetic is a compromise. Check the spec. advised for your car though.