Autodata gives 3 options according to climatic conditions, Cold: 5/30 Moderate: 10/40 Hot 20/50. Alternative, 15/40.
Now I followed that and put 10/30 in it (2nd number having nothing to do with the weight of the oil) although I was more interested to hear the opinion of many more experienced and knowledgable opinions rather than just follow what autodata says.
PS, checked the almost empty top-up tub in my boot.....15/40.
Would mixing 10/30 and 15/40 cause it to go tits up?-No
5/30 high speed mutivalve engines
(Twin cam 16valve and turbo lumps, be aware using fully synthetic in high mileage engines that haven't used it before=carbon removal and problems)
10/40 semi for general 8 valve rubbish and v8 's (not too thin or thick)
15/40 for worn engines-if it uses 10/40 oil-use 15/40 40 reduce consumption
20/50 for something like old mini's etc
(but cheap 20/50 will cause black death sludge in high speed modern engines)