I have a 2002 Ford Explorer with the 4.6 it has three hundred six thousand milesI put STP oil treatment stop leak in it I think the rear main seal might have been leaking it has since stopped from what i can tell the question is I'm using 5 W 30 Havoline with the STP oil treatment stop leakwould that make the oil too thick

 

The only thing it will affect as far as viscosity, would be the cold flow. However, the amount added (1 can) would be negligible At normal operating temperature, you would still have a 30 weight oil..

 

Wander what the cold flow be like a 10w

 

There's on average only about a 1% difference in cold flow between a 5W-30 and a 10W-30 motor oil.