Here in the U.S. it's 3 in 1 Oil by brand name. Actually any light oil such as used on sewing machines will work. I imagine the ubiquitos WD-40 will work just as well...
Seriously, I agree with Clanad - my other half uses 3 in 1 oil. Not seriously - being as though they say WD40 works well on arthritic joints, it's bound to be OK for hair trimmers, isn't it?!