The manufacturer suggests washing in cold water + mild laundry detergent, air dry only - the gloves are supposed to be machine washable. That might work for light stains and use, but mine get so nasty, I don't dare put them in the washer or even in my sink.
What I've done, seems to work well on grease covered gloves - take 1 part Simple Green, 3 parts of hot water - enough to soak the gloves in a bucket. Every once and a while, give them a swirl with a painter's stick or similar - you'll be shocked by how much stuff gets pulled out of the gloves. After they soaked, I rinse them out and give them a quick hand-wash with regular laundry detergent to get rid of the Simple Green odor - air dry, gloves are good as new.
All this assumes that they are the standard Mechanic's gloves. If there is any natural leather or suede on them, can not wash them in water at all.