Considering environment and temp only, when would you pick a 40w over a 30w?

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Jan 1, 2003
Not considering engine differences or driving styles, but simply the environment in which a gasoline vehicle is operated, under what conditions would you use a xw-40 instead of a xw-30? Variables to consider: air temp, humidity, length of hot season, cold season low temp.
If you live in an area that goes over 100F a lot, then definitely you should consider a 40wt, however it truly does depend on the engine. With some engines you might be better off simply going to a higher 30wt oil, something around 11.5 to 12.0. I think the engine design, and mileage, definitely play a big part in which viscosity works best. I think every engine has a sweet spot which shows it's best wear for instance. So when it's new that might be an Xw30 oil with a viscosity of 10.5 to 11.0cst. But at 100k, this engine might then be better off in the 11.5 to 12.0 range. This is exactly why the high mileage oils are formulated a bit thicker, and why oil consumption goes down when you switch to them. At some point this year I definitely want to experiment with running a slightly thicker oil in my Firebird, but it will be something that is either a very thick 30wt, or very thin 40wt (between 12 to 13cst, give or take) As always, I'm changing my mind ten times a day as to what tests I want to run, but I may consider 10w40 Pennzoil (which is formulated at 13.5cst, so it's a thin 40) for a short run at the end of the summer.
All year round in almost all climates. 5w-40 or 0w-40 (syns). A 40 weight is a medium viscosity IMHO, 30 weights are thinner and traditionally PPL only went down to 30's, to make thinner starting viscosity like a 5w possible. Nowadays we can have 5w and 0w with a 40 weight and no excessive use of VII's. I see no reason to go down, to a 30 weight in just about any climate short of a northern winter. Fred... [Smile]
Ken, Means nothing for thses 2 treasons: 1) Among non synthetic multiviscosity oils 10w-30 is the among most stable, since manufacurers assume you dont use syns, in their view a 10w-30 is the most stable oil viscosity you are going to use (dont ask about those (like GM) recommending 5w-30, that makes no sense, no matter which way you turn it [Roll Eyes] ). Also if you look at the graph, there is no one area where the 30 weight is more applicable than the 10w-40 except that the 10w-40 ranges up further (as one would expect). Thre fore one can safely surmise that the prfrence is simply there so they can measure their fuel economy based on that. Fred... [Smile]
Patman has it pretty well. We (and Ford) use 5w30 here in all Fords and Mazdas, and we have ambient temperatures over 140 F. I have seen a new Explorer where they put SAE 40 oil at 1000 km for 3000 km and it could never go back to 5w30 without consuming 1 liter every 5000. I had to increase it to 15w40 and it's now up to 25,000 km and the owner is happy again. Once the engine gets some wear and starts consuming, I increase one notch. My 97 Mazda was originally owned by the Amalia distributor, who had no 5w30 so he put 15w40 in it. When I got it at 125,000 km it drank oil. I increased to 20w50 and ended consumption. This week we put 5w30 in an Eclipse that was using 20w50 and the owner came back for 5w30 for his Ranger. Says he could feel the power difference. Bottom line: It depends!
My thoughts are that 10w30 is preferred is due to fuel economy and performance, not wear protection. I've been using 15w50 & 10w50 Group IIIs & PAOs in my camry for the last 3 years / 59000km in Singapore's hot tropical climate, and had virtually no engine wear. I believe the 50wts offers more wear protection especially if you tend to rev to 6000rpm often.
I could use a 5-40 for ALL TEMPS. above any/all engines...any/all the time.
Ken, what year is that Suzuki diagram from? My girlfriends's 1997 X-90's owner's manual is lost...wonder if that chart applies to her car??
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