Is it unusual for the car to function better with a thicker grade of motor oil? I was running Mobil 1 15W-50 with the AC Delco Duraguard oil filter in my GTA, and my oil pressure was always between 45-65 PSI at any given time, and would only drop to 32-35 PSI at idle during a heatsoak period (assuming 65 PSI because the oil pressure gauge would constantly peg out when cool and moist outside). However when I switched to Amsoil 10W-40 and their Super Duty Filter (SDF25) after running a bottle of Engine Flush, I noticed that my performance and heat endurance has really gone to **** with this new oil. The car really chokes when it gets spells of heatsoak, and I see oil pressure drop as low as 25 PSI. I referenced my maintenance log, and it was like this even before the switch to synthetic oil a few years ago; if I run lower than a 50 weight synthetic oil, performance and endurance in high heat situations really suffer.

If this helps, I also recently replaced my stat with a 180* stat, since the store didn't have any 195* stats in stock.

Is this strange? Any ideas? My car's in the sig.

 

the thicker the oil, the more resistance it creates, so the higher the pressure reads.

also, on higher mile motors, the clearances are larger, so while a thicker oil will show normal pressure, regular oil will show low pressure.... also thicker oil may quiet tapping lifters, and other noisy things...


this is why i always check the viscosity of the oil before i buy a used car.... just pull the dipstick and look at it...

 

Is it better for older engines to use thicker oil then? I'm sorry, I'm still learning.

 

Yes it is VERY unusual for a car to perform differently with different weight oil in it. While what the guy above said is true about oil pressure vs. viscosity, oil pressure has NO bearing on how an engine runs as long as it has enough oil to keep the lifters pumped up.

In other words, unless you're doing back to back dyno or dragstrip runs, i think the performance difference is in your head. There is NO mechanism for an engine to slow down because of thinner oil. In fact thinner oil is usually worth MORE power on a dyno because it's less work your oil pump has to do. I run 5w30 year round in my car because i figure it might be worth 3hp and i could care less if the 305 blows up anyway.

As for cooling problems with a thinner oil, i think you're barking up the wrong tree as well. While oil is the primary coolant of the bottom end, if it was actually thinning enough to break down and stop lubricating things in heat your engine would have blown up by now.

The imagination is a powerful thing. There is no greater liar than the butt dyno. Measured data is the only way to make comparisons, and you have to be careful drawing conclusions from incomplete data.