On my new purchased 855 with 150k, I do not have much history. I am a big fan of Syntetic, but since i am not sure what has been run in the vehicle, should I be concerned? The Rear Main is already leaking and obviously will be fixing this leak, so I was wondering if it then makes sense with the rear seal being changed that I should start out fresh with Synthetic??

Unless you plan to go to longer oil change intervals I can see no benefit.

What if..... The vehicle has been running on synthetic?? Remember I have no history..

In my opinion that is not an issue. Synthetic, semi-synthetic or dino oil. Switch to whatever you like. the dino oil should leak a bit less, you could also consider one of the high mileage oils. My suggestion is Castrol GTX, or Shell Rotella diesel oil.

I have never used synthetic, but I've been fairly good about changing oil after 3,000-5,000 miles, and my engine with 150,000 miles is like new inside.

Make sure the PCV system hoses and fittings are clear before replaceing the rear seal. There's also a hose connection on the throttle body flame trap that often gets plugged and needs drilling out. You can try easing into a full synthetic oil engine by just adding one or two quarts to the first few oil changes and gradually increasing the concentration. But at your current mileage why spend the extra $$ for synthetic oil? It's really not going to benefit you very much. Just use Castrol and a good Mann filter.

Replace as many PCV components as possible otherwise the leak will return. Do not install a flame trap. Don't worry about switching to synth. Just do it, and change the out the first batch after 1000mi or so. Then every 5000 after that.

97 855na 170k two rear main seals done