My haynes manual suggests that I use 15w/40 oil in my petrol-340dl-1.4 and Ive discovered Asda sell 5litres of it for a tenner but it is diesel mineral oil. does that matter?

i think 5w 30 or 5w 40 is better, thats what im running on mine (5w30)
and most oils can be used on most engines
dont always go for the cheapest brands of oil btw.. the more expensive ones are that because of a reason, i go for castrol instead of mpm

I tried marine diesel 15/40 once and the engine wasn't very happy with it, judging from the noise it made at high revs! I think 20/50 is better for an old engine and not much dearer from the cheapie shops. Bests, bogbasic.

15W/40 would be fine if it was semi-synth. I don't trust mineral oil. It is however perfect for your gearbox.

Too late I've just replaced my cars oil with 4l of asda's own brand diesel 15w40 mineral oil, car seems to be running Ok...

Is it worth me replacing it again with castrol gtx 15w/40 part synthetic oil?... I dont want to ruin my engine...

Just listen out for it getting loud. You might also have to change it sooner... 3000 miles maybe?

Just talked to a local reputable mechanic and he says its fine to put mineral oil in my car and that part synthetic wouldnt have been around when my car was originally manufactured in 1988 anyway...