I just use a 40 degree wash if its muddy or 50 if its really muddy, then gets whacked straight in a dryer afterwards as i normally need it the morning after.
My gear is normally covered in oil, grease, lots of paint and anything you would find at a site, The pants i use are now 2 years old being worn upto 4 times a week and still look pretty good,
1 thing i have found cheap nasty soap powder is actually very good at bringing mud out compared with things like daz, bold etc. (we wash upto 200 pairs of overalls a week in the stuff and there not stained) so all my kit gets washed in asda or tesco own brand soap powder.
Jon