intermittent excessive oil consumption
 

I've got a 98 e350 powerstroke 7.3L diesel engine that is really confusing me. Sometimes it seems to use a ton of oil in just a few hundred miles, and sometimes it doesn't use any. There is a leak at the dipstick tube insert on the oil pan, which i tried to silicone up, but didn't entirely work, but that stll doesn't seem like it would account for 2qts every 300 miles. It's not showing any smoke at all in the exhaust, there is some smoke in the crankcase when you open the cap, but i figured that was somewhat normal. Anybody got any suggestions?