First, the oil pressure gauge on Fords are not real pressure gauges, in the mid-1980's Ford changed them to an oil pressure switch sender on the engine, the switch grounds the gauge if oil pressure is above 6psi.
With key on the gauge power flows through a resistor so shows 0 psi, when oil pressure gets above 6psi, the gauge gets a Ground from oil pressure switch on the engine, so less resistance and will move up to about 1/2 way.
Any movement of the gauge's needle after that is strictly electrical, i.e. higher RPMs can increase alternator output and voltage in the system, so gauge can move up and down with RPM but usually doesn't move much, no movement just means your voltage regulator is working as it should, lol.
The oil pressure switch sender may be going bad if you get pulsing of the needle, I would get a new one, they are not expensive or hard to change.
Yes, 5w-30, "acts" like a 5 when cold, so easier to pump through cold system, and as it warms up it becomes 30 at 210degF, operating temp of gas engines.
So 5w30 and 10w30 are the same when warmed up.
5w20 would be thinner after warm up, a 20, but the same cold as 5w30, a 5
So assuming your engine runs at the normal 200-220degF after warm up and 30 is what you have been using then I would stick with that.
If your engine temp gauge runs lower in winter then I would get a better thermostat or put a piece of cardboard in front of rad to slow down airflow in engine compartment, in cold weather the engine can bleed alot of heat at highway speeds because of the airflow through the engine compartment, so thermostat stays closed but engine still can not stay at 200degF because of the air flow.
I know the 4.0ls suffered from this as well as the 4cyl engines in colder climates.
Ford uses a 192degF thermostat, or 195degF, either should be used, some replace with 180degF, bad idea, only costs you money in extra fuel needed, doesn't help engine to "run cooler", that is a myth.
Switching to 5w20 in winter is common but IMO not needed for colder weather, and it would have to be extremely cold, then 0w30, the 0w is the cold viscosity.
Slow cranking is a sign of weak battery(they lose cranking power as temp drops) and thick oil, new car battery is 12.8volts, when they gets down to 12.2volts it's time to shop for a battery sale, usually they are 5 to 6 years old at this point.
After startup issues would not be oil related, outside of loud ticking in valve train, lol.
Last edited by RonD; 11-13-2014 at 09:16 PM.