Is thicker oil always better?


Hello. We own a Smart car and they tell us to use Mobil 1 0w40 oil, but wouldn’t a thicker oil offer better protection? Several mechanics that we spoke with have told us NOT to change the oils viscosity, because they have heard about people that have had camshaft and other engine damage to their engine because the thicker oil can’t reach the tight tolerances of these newer cars when the oil first starts up. Are they just trying to scare me or should we really use the 0w40 oil?


Read the manual, see what it says. If it specifies 0w40, then use 0w40. I suspect the engineers who designed the car know a thing or two about what’s good for it. Why don’t you want to use the specified oil?


0W40 is pretty thick (40 weight ) oil in the first place with a temperature rating of 0 degrees. Do what the manual says because Ive seen trucks ruined by wrong oil. eg.Ford pickup required 5w20 oil, old timer buys his usual 10w40 and lockes the engine. Why? because the tollerances built into the main & rod bearings would not allow the flow of the heavier oil into the miniscule space between bearing and crankshaft. The newer tech engine ( 4.2l 4.6l ) has an entirely different set of specs from his older 5.8l .


It’s not that I don’t want to, it’s that I always thought that using a thicker oil is better. Are you agreeing with the other mechanics who have told us not to use anything else other than what SMART CAR recommends? I just want the best protection for my car. I also heard that using a thicker viscosity oil will hurt the Smart cars fuel mileage. Is that because there would be more unneeded torque placed on the engine? Thanks for your help with this.


Use the oil required by the manufacturer, thicker oil does not necessarily provide more protection. Older engines with less precise tolerances sometimes required higher viscosity oils, especially at higher temperatures.


Thicker oil is not always better. The optimum viscosity has to do with engine tolerances and sometimes with excessive engine wear. For your new SmartCar, go no higher than the reommended 0W-40. That stuff they told you about tight tolerances is quite true.


It’s not that I don’t want to, it’s that I always thought that using a thicker oil is better.

No. If that were true, then we would use 90W gear oil, or even wheel bearing grease.

If an oil is too thick, it causes additional load and reduces mileage, it also will not move into the smaller areas to provide lubrication nor will it move as quickly through the engine where it provides cooling.

On top of that today's oils are far different than they were when I started driving (mid 60's) Modern thin oils will provide more pressure protection than those old heavy oils.  Stick with what the manufacturer recommends.  Failure to do so could cause damage to your engine.


I wanted to thank all of the kind people for responding here. You guys are great and it was nice to get some honest answers on this topic from people in the car industry. I will use the 0w40 oil as Smart car recommends. Thanks for all of your insightful answers :slight_smile:


“0 degrees”??? No it’s 0 weight AT 0 degrees F (and 40 weight at 210 degrees).