You can certainly tell if the turbos are leaking oil by looking inside the pipes, though it sounds like valve seals to me. When you let off the gas the engine has the most vacuum and would likely pull some oil past the seals, if it was turbo it should do it under boost too. That AFR is a little too rich for me not to fix it, you start worrying about cylinder wash down at that point, its not an imediate threat but it will shorten the life of your engine. I'm not a Supra guy so I don't know specific fixes for your car and what they envolve, but I'm sure somebody can provide some good input on that.