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 over time) 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. But during cold morning starts, I does 'feel' (while driving off), as if the engine is already 'warm' and the oil is thin. It accelerates a bit smoother.

i have always heard the opposite of what you found. sounds interesting enough to give it a try though

i have been taught to keep the engine 180 ..
you may run better ..

I agree I would like to see temps a bit higher again near the 170F range. Especially when outside temps start to drop aswell. So I will install a 180 thermostat again somewhere soon.

For the moment I'm happy with the cooler temp as the motor has a rusted thin coreplug at the rear. With lower temps the coolingsystem builds a little less pressure and along with the 7 lbs rad.cap it will hopefully save this plug a little longer till I'm ready for my planned overdrive tranmission swap.

Since I went to aluminum heads on my Dart and truck, I have to be at 180 when I leave the line. 160 or lower and it will lose a tenth. The dyno confirmed that on the truck engine as well. It was down almost 15 horsepower at 160 degrees compared to 180. Both pulls were with the oil temp at 160.

I would think with the heavier oil you were working
the engine harder... driving the oil pump and the added
force on all the bearing with heavy oil...you have to
press the pedal a touch harder which is fuel which is
temp.... but I wouldnt have guessed 15*...I picked
up a bit when I changed to syn oil.... JMO

From what I have read a semi synthetic is mostly dino oil, there is not set amount of synthetic it has to have to be called a semi ???

What probably happened is this: Your original setup pumping a higher viscosity oil needed less oil to the bearings because it wouldn't flow out of the bearings as fast; this caused your oil pump pressure relief valve to be operating most of the time, dumping oil back into the pan. Lowering the viscosity meant more oil could flow out of the bearings, less or no oil came out of the pressure relief.
Oil coming out of the pump is warmer than the oil going in, so bypassing some of that warmer oil into the pan raised the temperature. Stopping the bypass flow eliminated that source of heat.

R.

Sounds plausible.

JohnRR,
The oil is a Kendall GT-1 '5w30' (API SL). The can said 'Synthetic Blend'.

It is my believe all the base-oils are the 'dino'-stuff, but it's the type of additives that define if it's a synthetic or 'regular' oil.

That's not correct. In the oil world, base stocks are defined by their "Group number". Petroleum oils start out with Group II or III, while synthetics start out with either Group IV, polyalphaolefins, or Group V, which is composed of many different types of base stock that had no place elsewhere.

"Oil Base Stocks
www.zddplus.com/TechBrief10%20-%20Oil%20Base%20Stocks.pdf · PDF file"

Synthetics have a flatter viscosity curve per temperature, so they need less Viscosity Index improver or none at all. VI improvers have a tendency to shear and lower the viscosity at higher oil temps, which in the '80s led to engine failures.

R.