Grease-cutting solvents like muratic acid are quite harsh, and any we suggest will probably damage your lawn. Just for fun, pour some Coca Cola on a small part of the stained area. Make sure that the area doesn't dry out for two or three minutes while you scrub with a bristle brush. Absorb the excess cola with sawdust or cat litter. Remove the absorbent material with a broom and dust pan, and hose down the area with fresh water.

Cola has been known to remove oil stains, but it's not a guaranteed method. Muratic acid may be your only alternative. If that is the case, you may have to dam the area between the walk and the lawn. Cardboard joined with duct tape should work.