20w50 is pretty thick. i'd suggest running a can of engine flush through it, its really mainly keroscene. the stuff actually works, i was skeptic untill i've torn down a motor after doin it. and i've seen it fix noises i was makin fun of guys for tryin the stuff in. run that stuff in there for 5 mins. or whatever, drain oil for as long as you can, let it drip all day if you want, change filter, and add oil.
i am a fan of lucas, i had an old xj w/ 160k that used a qt. a month. after flushing, changing oil frequently, and using lucas, it didn't use a qt between changes. and on top of that, once it stopped using oil, i didn't use lucas either, and it never used a whole qt between oil changes.