I own a Peugeot 405. At first I was using 20w-50 engine oils but The last time I went for changing the engine oil I used 10w-40. After that The engine temperature rises constantly and (even when it's cold outside) and after the second fan starts it'll cool down immediately.

I check radiator water constantly and refill it if needed.

Can the engine oil affect engine temperature?

  • Could you give the exact model and year of manufacture?
    – Zaid
    Apr 5, 2016 at 8:59
  • @Zaid Actually the thing is that I live in Iran and the company, IKCO, is rebuilding and modifying cars from Peugeot and formerly Citroen. The car's model is Peugeot 405 2012 and it uses XU7JP/L3 engine which also burns CNG as fuel! Apr 5, 2016 at 9:06

4 Answers 4


I don't think so

I've had many experiences with strictly oil cooled engines. In particular the 1986 Suzuki GSXR 750/1100 platform.

They went strictly oil cooled with a giant oil cooler and 8 quarts of oil.

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As you can see, this motor is filled with oil galleys and has no coolant or radiator.

Another note, I live in the desert of the US Southwest where temperatures exceed 120F in the shade during late June to August.

I have never had an issue running 10w-40, 20w-40, 20w-50....so on. Although the 10w-40 is 'less dense' than 20w-50, in theory, I have not detected a difference in power, cooling or engine life.

Certainly, comparing this engine to your liquid cooled modern car platform might seem ludicrous but at the end of the day taking the motor I am talking about to a track and running it a very high RPM's in desert summer heat and not detecting a difference in cooling is pretty meaningful.

You mentioned

I check radiator water constantly and refill it if needed.

If you are needing to refill your coolant on a semi-regular basis I suggest you focus on your actual cooling system.

I haven't added coolant to my 2013 Toyota Truck for at least 6 months and I'm sure I'll be able to get to summer before I add a little to the reserve reservoir.

IMO, focus on the actual cooling system.

  • The water in the radiator goes down every other week, as I asked my mechanic it's normal for Iranian cars made by IKCO. Apr 5, 2016 at 9:09
  • It will not reach the danger zone in temperature (red areas) but I'm a little worried it might cause a problem, since we are getting close to summer. Apr 5, 2016 at 9:10
  • Your GSXR is air-cooled. I don't know if the OP's car uses an oil-water intercooler but that could explain what he's observing. If I had some oil properties I could try to predict what temperature differences one could expect from using 10W-40
    – Zaid
    Apr 5, 2016 at 9:50
  • It's not really air cooled. It's 8 quarts of oil. I made it pretty clear that the bike was not liquid cooled. Apr 5, 2016 at 16:30

The only place where I expect direct interaction between oil and the cooling system is the oil-water heat exchanger.

I don't know if your 405 has one (some oil-coolers use air instead of water) but if it does have one and the 10W-40 runs hotter than 20W-50 (I don't know how to verify this), this might explain the hotter temperatures seen in the cooling system.

  1. Check/ replace water temperature regulator ( if the regulator is late to open, the coolant temp sensor will be late to order your fan to start, hence a higher temperature)
  2. Make sure their is no air bubbles or trapped air in the cooling system ( air bubbles can reduce heat transfer in the cooling system hence over heating)
  3. Use manufacturer recommended oil viscousity ( usually found on the oil cover or/and operator manual) Please, it is not normal to that you add a liter of water every week. Note: tap water is not a replacment for coolant. Check manufacturer recommendations for the coolant mix.

The level of oil will definitely make an engine run hot (how full is it?). If you are having to regularly add water to the radiator, you should check the oil quantity and condition at the same time. I'm very skeptical that oil viscosity would make a noticeable change in engine temperature all other things being equal.

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