Well, 10w30 is a good oil to use. I use 15w40 personally, but would use a 10w30 in it too. 20w50 is a bit too high for winter starting, so I would stick to 10w30 as anything much thicker will not work properly in our cars.
AND DO NOT USE A SYNTHETIC OR A SEMI-SYNTHETIC OIL. The only synthetic oil the BMW manual suggests to use for our cars is in the rear differential (Castrol SAF-XO). The manual states that you can use special oils (synthetics are special oils) only if you need to but to replace the oil AS SOON AS POSSIBLE! I have done a lot of research into oils and have no good reason to use synthetic oils, and when reasons are given, all I'm given are the marketting lines (it lasts longer and protects your engine, etc, etc) and not any chemical composition or scientific reasons.
As well, where is the oil leak coming from? Also, what year and model do you have?