I changed the oil this week in my daily heap, the '73 Dart.
I went from a regular dino 10w40 (with some ATF mixed in) to a synth. blend 5w30. Both oils were rated API SL.
Decided that even in the summer, the oilpressure was good enough to warrant a thinner oil in the engine.
First thing I noticed the oil pressure was pretty much the same as before; Oil pressure is 60-65psi cold idling and cruisin, and between 35-40 psi hot idling in Drive.
Most interesting however was the watertemperature.
Normally the engine runs at 160-170F, but now since the oil change, the temp hardly gets over 150F anymore in and around town.
Only on the highway the temp climbs back to 170, but that's because the radiator is pretty scaled up inside. (There is a 160F thermostat in the system.)
Does the newer oil take up engine heat that much better so it runs 15F cooler, or does the engine feel that much less friction and drag of the previous oil?
I haven't got a tach in the car so I can't say for sure the engine runs faster now.
During cold morning starts, I does 'feel' (while driving off), as if the engine is already 'warm' and thin. It accelerates a bit smoother.