Hello,
I have a 1993 FD that I've owned for about six months now. It's all stock under the hood with only an aftermarket exhaust.
Today when I changed my oil, I switched from using Castrol GTX 10W30 to Mobile 1 15W50 since after searching around the forums I saw that a lot of people were using/recommending a thicker oil such as 15W50 or 20W50, so I thought I'd give it a try. After the change it is now idling at around 600 when fully warned up (around 520 with the headlights on). Before the change it would idle at around 720. Is it normal to experience a lower idle when switching to a thicker oil? Also, is it bad for the engine to idle that low and could this be a sign of a problem with the oil pressure?
Unfortunately I don't have an aftermarket oil pressure gauge installed yet, so all I have to go off of is the stock one. The pressure is reading normal levels and is no different than before the oil change.

 

Interesting...

In my TII I used to switch between GTX 10W40 in the winter to GTX 20W50 for summer races and I never noticed any change in idle speed.

However, I had many oil mods to lower the load on the oil pump so it wasn't straining super hard like the stock pump...

If anything it would be increased load on the oil pump from the thicker oil that could lower idle speed.

But, I think you might have done something else by accident when you changed the oil. Tuned off the AC?

 

your idle shouldn't be affected by viscosity to that degree, i agree that you probably inadvertantly bumped something.

if your car was modified and had a super lean idle tune, i could understand but not a stock-ish car. you'd be lucky to even notice a 20 RPM difference.

 

All else being the same I don't believe the change in viscosity is responsible for lowering the idle. And with lights turned on or any larger electric load...as well as turning on the a/c, the idle speed should go up, not down.

 

Yeah, that is why I asked if he turned the AC off when he noticed idle was lower.

 

So I figured out what the problem was. I went back and looked at anything I might have touched and found that an electrical connector right above the oil filter was loose. Plugged it back in and it idles fine now. Must've bumped it when I removed the filter.

 

Sounds like the IAC (Idle Air Control) valve.

 

Glad to see there was a logical explanation, and not oil weight changing idle speed

 

maybe if he used honey instead of oil.

 

Omg, i can't imagine pouring 5 quarts of engine honey in, that **** take all day

 

stick em in the microwave first!

 

What? Then keep halogen lights on the oil pan so you can start it when its below 80?

 

 

oh i know it exists, i use a mix of STP motor honey and 20W50 for assemly lube.

 

changed oil from rotella t4 15w40 to 10w40 with zinc additive last night. this morning i went to work and the thing didnt wanna idle. i accidentally let it bog down and it stalled on the highway when i came to a stop. got it pushed to a parking lot and the thing didnt start until 40 minutes later when it cooled down. didnt get in trouble at work for coming in late. adjusted the idle on break. drove home just fine. today was a mix of emotions i thought i blew another engine.