You need to find another mechanic.
He found a code that said that it was running lean. He made some slightly off the wall suggestions to try first and then he wanted to replace the oxygen sensor.
Most of the time, oxygen sensor codes for running lean are not caused by the oxygen sensor. Someone needs to find out why it is running lean, then you can figure out how to fix it.
Since it comes on only after getting the car up to 70 MPH, it isn't likely to be a vacuum leak. It is very likely a fuel delivery problem. A fuel pressure test while driving is in order. It would not hurt to change the fuel filter first to see if it helps if the filter has not been changed in a while.
By the way, it would help if you told us what year the car is, which engine you have, roughly how many miles it has.
