30 or 40 weight oil

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10
The owners manual for my vehicle lists multiple viscosity options, but 2 of the 3 are 30 weight oils. The other 1 is a 40 weight oil for higher temperatures.
My question is does it make sense to use a 40 weight oil when it's hot, because when at operating temp it shouldn't require a thinner or thicker oil depending on temp, right? Should a 40 weight be used when putting the engine under more demanding conditions, such as towing or regularly driving up hills?
What is your guys' opinions on when to run the recommended oils?
And as a little info on what oil I usually use, I have used 5w30 oil for most of the cars life. I've also used 10w30, as recommended by the manual in hotter Temps once. And I've use 0w30 oil even though not recommended by the manual because as to my knowledge, the lower start up viscosity the better... shouldn't hurt anything, should be better to my understanding. Since the oil is a 30 weight at operating temp all is good with that I suppose depending on you opinions on whether it makes sense to use a 40 weight oil at all, would a 0w40 oil be a good option as well as the manual recommended 10w40?
 

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Subaru Baja? Non turbo is fine with a 5W30 while from my experience as a former WRX owner, the 2.5 turbo should really use something that is VW 502 approved.
 

Jetmekanic

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So its a 2003 Subaru Forester. It is a daily driver for now and sees some overlanding/offroading on weekends and occasionally week-long trips in the backwoods
 
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2,208
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Muncie, Indiana
So its a 2003 Subaru Forester. It is a daily driver for now and sees some overlanding/offroading on weekends and occasionally week-long trips in the backwoods
So it's only N/A, if it makes you feel better go on ahead and dump the 0w-40 into it, it's pretty much a heavy 30 grade anyways, Castrol A3/B4, Mobil1, Valvoline, Pennzoil euro oils would work just fine in the engine.
 
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Massachusetts
So its a 2003 Subaru Forester. It is a daily driver for now and sees some overlanding/offroading on weekends and occasionally week-long trips in the backwoods
As an avid offroader, I don't see those conditions to be very severe. I would just keep running 5W30 and enjoy the camping trip.
 

Jetmekanic

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As an avid offroader, I don't see those conditions to be very severe. I would just keep running 5W30 and enjoy the camping trip.
I agree, it's not usually very severe. Though I have gone and done things that were a bit severe for the vehicle. I can't monitor the oil temp, but I have had the transmission over temp light come on before
 

Jetmekanic

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Thanks for the replies. What I gather from you guys is that any of the oil weights mentioned should be just fine. I'll watch for anyone else's opinions, but for now I think I'll probably just keep using the oils I have been.
 
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