On 98 V6 at what mileage should it be changed. I have 170,000 miles now. Always run full synthetic.

 

If you have the front apart, change it for the low cost.
If not and your concerned check the loosness by by rocking the crank at the front pully to see how much slop there is against the chain before the cam gear moves and/or the valve train moves sleightly. Were talking like 1 or 2 degrees at the crank, max.
I would think that at 170k there would be some wear and strech.
There are 2 gears and the chain to wear so it does not take much to add up to an excess at the crank, for movement.
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What the excess wear does is make the Cam timing 'a bit late' or retarded in relation to crank position.
This tends to cause some loss in low end torque. Of course fuel injection timing change goes with it being geared off the camshaft.
It's a judgment call.
Good luck