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Also make sure it isn't the front crankshaft seal, which oil gets "blown" back on the oil pan and can appear to be a rear engine leak. LS engines are notorious for wearing out front crankshaft seals and balancers.
Did this mechanic crawl under the car or just guess? Only way to tell is a close inspection underneath and up close. Even then it can trail from somewhere else ending up on the low spot and look like it came from that area.
shop performed an oil change and noticed the issue....they suggested injecting dye to more accurately pinpoint the leak.....and i will do that after my monitoring for a few weeks.
Make sure you get it checked by a true mechanic and not just a oil change place.
This fix can be very costly, at the very least the whole torque tube needs to be dropped in order to access the rear main seal - it is not a slam dunk easy job!
It would be prudent for yourself to go under there and check too, keeping looking for the highest point of oil wetness, as stated the oil sending unit can drip down the back of the engine and "APPEAR" to be coming from the rear seal
Get a good high powered light, and you can even get a DIE KIT from the local auto store and do it yourself - it comes with the light too!
Let us know what you find - and if you don't have the jack, jack stands etc., enlist a friend who does and give him some beers for the help!