Could be a bit late for oil mate.
When the light comes on you will find that the sump will almost be empty, as unfortunately the light only tells you that you have no oil pressure.
Fully synth 5W 30 is what you want for a 350.
Ideally same tyres all round is what you want, however mixed as you have suggested will work, but not ideal, especially with winter almost upon us.
As has been suggested fit a complete set of MPSS and sell your part worns.