What actually affects oil temperature?

[Trix]

My WRX has an oil temperature readout. I usually keep this up so I know when I can safely get into boost at temp. I recently switched from the seemingly excellent Pennzoil Ultra to Redline 5w30. Just some experimentation, possible track day soon.

I have noticed a decent drop in oil temperature of 10 - 15 degrees F (214 max vs 224 or so). I was actually expecting it to go up a little bit, with more intermolecular 'friction' , this is a 3.7 HTHS oil, vs the ~2.9 before. This got me thinking though, what actually affects oil temperature, what does it mean? Keeping the car and environment constant, what can we infer about a temperature change?

Is there actually less friction in the engine? Is the conductivity of the oil less? Is the heat capacity of the oil less? Does the temperature sensor not read some oils as well? Both of these oils were well within safe range, so I am not worried either way. To me, either the heat isn't there, the heat isn't making it into the oil as well, or somehow the heat is being dissipated more? Granted this is relatively small difference. But hey this is a crazy enthusiast forum.

 

[Rand]

my fb25 hit over 236f in regular highway driving.

I'd say the difference is statistically too small to narrow down to anything relevant.

 

[Ducked]

Not sure how "statistics" apply, or could be applied. IF its consistently different, then its a real change.

On the face of it, I'd be concerned about the implications for your previous oil. If a higher viscosity is running cooler it might imply there was more contact before.

 

[Trix]

Originally Posted By: Ducked
Not sure how "statistics" apply, or could be applied. IF its consistently different, then its a real change.

On the face of it, I'd be concerned about the implications for your previous oil. If a higher viscosity is running cooler it might imply there was more contact before.


Well perhaps, that is why I asked. What could and what would cause any difference at all. Not in my application but generically. I've changed my oil 4 times now, first time with a different oil. Factory fill was slightly higher, but it was break in, so not sure if I want to count that. It has been consistent for a while, until now.

 

[legandrex]

For whatever it's worth I did a track day in my 15 wrx at Road America and I believe the highest oil temps I saw was just under 240. I think I was running the old Ultra back then. I think I brewed it at 1/2 5-30 and 1/2 5-40. WRX oil cooler works pretty well.

 

[demarpaint]

How many miles were on the engine when you started running the PU, and how many miles were on the engine when you started running the RL oil? You say this is the 4th time you changed the oil.

 

[CR94]

Originally Posted By: Trix
...I have noticed a decent drop in oil temperature of 10 - 15 degrees F (214 max vs 224 or so). ... Keeping the car and environment constant, what can we infer about a temperature change? ...
Not much from a change that small, unless it happens consistently and you carefully controlled all the relevant factors (ambient temperature, wind, engine speed and load, oil level ...).

 

[Virtus_Probi]

Interesting...I have a Forester XT with a similar engine and noticed a roughly 10 degree rise in engine oil temps when I went from a 3.1 HTHS oil (M1 5W30) to M1 5W30 ESP with an HTHS of almost 3.6. I always have the oil temp up on my display and sorta freaked out when I first noticed temps I hadn't seen before...I was worried that the car was overheating! The oil temps went right back down when I ran either M1 or M1 EP 5W30 for my next change. I posted here about what I had noticed and some kind users pointed out that this was to be expected with a higher HTHS oil.

I'm really surprised that you saw lower temps with a higher HTHS oil and am curious to see if anybody has an explanation.

 

[Jetronic]

The ester content of Red Line will have more mass than the grIII synthetic used by pennzoil. You can readily find ester oils around 1 kg/dm³ while GRII is closer to 0.850 kg/dm³. It just takes more energy to heat that oil to the temperature and that's negating the speciic heat as I don't have any figures, nor know exactly which esters Red Line uses. But their MSDS is very indicative of a higher than normal specific gravity.

 

[rodinator1234]

What actually affects oil temperature is RPM, not engine load per say. The friction of the rod, main bearings and the pumping intensity of the oil cause the heat. What I mean by pumping intensity is hydraulic systems generate heat the more fluid that's moved through the system. At high RPM the pumping pressure, flow and intensity are all contributors to heat. The turbo also contributes to the heat as a result of RPM as well.

 

[KL31]

I have an oil temp gauge myself and it can vary quite a lot depending on ambient temps. You need to drive (my car) at least for 20-30 minutes to reach a stable temp. Longer in very cold weather.

Highway would be a great comparison. If you set cruise control and keep test conditions the same (RPM) then it would be interesting to note the difference. Having owned two Subaru vehicles, I can agree with most here that they do indeed like to run hot and are lightning quick to warm up.