Andy,
Not a bad idea to use a mineral oil for running in (it certainly won't do any harm), but I would try to find something that copes with cold better than a 20W, especially given that it's now winter. A 10W-40 would be much better.
For an explanation of oil grades take a look at:
http://www.brakehorsepower.com/speedtrap/engineoil_bible.html
Mike