Engine runs 15F cooler after oilchange (10w40 to 5w30)

[BigBlockMopar]

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?
Or both?
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.

[oldsman71]

went to 10w30 from 20w50 and it seemed a little quicker but be real careful if you get stuck idling in traffic!
I got stuck in a long line leaving a powertour venue and when it finally got hot, the oil presser wasnt so good
it didnt burn down. I dont think the thin stuff protects as good with looser bearing clearance. I think with tighter
clearance you should be ok. mines .003 rods .003 mains

[anotheridiot]

Friction is heat, so squeezing heavier oil is gonna create more heat for sure. as long as your pressure is high enough you shouldnt have a problem. Figure 10W-40 might have been a better step for Oldsman, almost sounds like a mix might work out, like 4 10W30 or 40 and a quart of 20W50 like STP.

[Barry Donovan]

atf has a frictionizer.
adding to oil must have been a plan related?

that sneaks up on regular cars with open pcv and leaky atf from power steering or tranny..
not good to have in most engines

[anotheridiot]

atf has a frictionizer.
adding to oil must have been a plan related?

that sneaks up on regular cars with open pcv and leaky atf from power steering or tranny..
not good to have in most engines

Well the frictionizer is what helps sloppy bearings and rings seal better in older engines. I think someone got the idea with hydraulic lifters, think marvel mystery snake oil is hydraulic as well.

[BigBlockMopar]

Well, about 'that plan'...
the ATF in the old oil was an attempt to thin the 10w40 oil slightly and help soften the valveguide-seals and seal-up the crank seals a bit more. I thought it was a swell idea... :o)

[oldsman71]

Friction is heat, so squeezing heavier oil is gonna create more heat for sure. as long as your pressure is high enough you shouldnt have a problem. Figure 10W-40 might have been a better step for Oldsman, almost sounds like a mix might work out, like 4 10W30 or 40 and a quart of 20W50 like STP.

thought maybe to add some lucas to my next 10w30 or 10w40 oil change