Idle issues

i live in a state that is very cold however this issue just started about 3 weeks ago. I honestly do not feel like paying 130 bucks a hour diagnostic charge to the toyota dealership either when it could be a easy fix i can do myself. the issue is my engine does have a little rattle/knock when cold like valve clatter almost but it goes away once the engine is warm, that however is not the issue as of now. car has 93k on the clock on a 2007 model year. after the engine is warm and your driving the ICE idle is high and gradually comes down its not instant like it used to be. you can be driving at 50mph and you can just hear the engine running when you never used to. my mpg is suffering as well. usually i would get about 45mpg and now im down into the high 20s to low 30s. the only thing that i had that could possibly known to cause a problem was the oil was replaced 5k miles ago with castrol synthetic 5w-20. after 4000 miles it had started to use the oil so within 1000 miles i had to add one quart, but i replaced the oil with mobile 5w-30 and so far the oil level is fine so im assuming that was a fluke but could that old oil have gotten past and gummed up the plugs, or the throttle body?

 

thanks patrick. ill try that next month when i get paid again. do i need to avoid getting the cleaner in the holes below the butterfly plate? i can always use a old shirt and stuff down into the intake to keep cleaner from getting down in there and messing things up more. the plate is a little dirty but nothing to bad. but that probably dosn't say much about the gunk around the butterfly plate where it moves. it is a little hard to turn i know that.