Phil - I always use petrol but be careful not to cause a spark and use a stiff brush/old toothbrush/small flat bladed scraper to shift the really hard crusted stuff, a lot of which is often around the diff. Once cleaned off of oil, I then go over the steel bits with an angle grinder with cup brush to get all the rust off and provide a nice base for a coat of paint. If the Coxes are rebuilding your diff, get them to put a drain plug in the bottom of the casing - if you want it to look the dos do-dahs, get then to powder coat the front and rear mountings to get it looking like the one in the pho