Here is my Bullitt's pull on a Mustang Dyno, no modifications. We hit the speed limiter in 4th gear at 6700rpm.
View: https://www.youtube.com/watch?v=aZ1ZaWOEu8I
Best I got out of my '88 with aluminum drive shaft, long tube header, and 4.10 gears was 18.09 in the 1/4. Dual plug head would be in the 17's and it still ain't cool.
I'm willing to bet it'll go away once the 02 is put on. The computer has to make changes, even at idle, when you put the car in gear, assuming the transmission is automatic.
The oil return line is fine. The turbo ran good up until I fixed my miss condition. As soon as I hit 15 psi, smoke! But only for a few seconds after I let off from full boosting. Any other time it was fine. Now it smokes horrible at idle but not while cruising.
It's the aux. shaft gear. It's best to take off the distributor cap and set that gear where you want the number one cylinder tower on the distributor to sit by watching the rotor turn.