Those recommendations were written for the oils around when the machine came out. With modern oils you can get away with 15W-40 down to about 20F or a synthetic 5W-40 much colder. I'm not sure what spec they have in the UK (probably the same as on this side of the pond) but the CJ-4 spec is current over here and will work fine using fuel with less than 15ppm sulfur content.