i have a question about synstetic oil. would it hurt to run qakerstate 4x4 & truck synthetic oil in my 1990 f150 with the inline 6

 

nope.

 

Is the F-150 not a truck????
There are even 4x4 F-150's with 4.9's...

All kidding aside, as long as it's the right oil(10w30)....then no.
Some have concerns about switching higher mileage engines to synthetic oil, as it can cause weak seals to leak, but not everyone experiences this. I suspect it has to do with the maintenance history of the vehicle.

Having said that.....Mobil 1 is a far superior synthetic oil