I actually thought (from what I've been told) that it's better to just let them out onto the grass in the afternoon, because by that time the sun has burnt the sugars out of the grass (to a certain extent) and during night time the sugars regenerate I guess you could say.
My tb is on extruded Barley, and he was on vegetable oil and that worked a charm (I've now taken him off it as he's getting to be a bit of a porker for now.. Not in work though)
Apparently the best oil to use is rice bran oil.
If you put him on the grass just watch his poop, as when I put my boy on rich grass his system didn't handle it well, and because his poop became loose (extremely) he lost more weight than he put on, so I had to combat that.. I halved his grass ration and did the rest with hay, and I also fed him soaked palm kernel(like copra meal I think) and that's all fiber so that worked too.