Yesterday the dealer serviced our WRX for the first time. When asked what oil they used, they had some trouble answering. Finally I was told "15w-40, SI rated, any brand you choose." They didn't know what brand was in their drum!
Last night I read that SI was a rating that was intentionally skipped, as it is a unit of measure somewhere. SJ is the latest rating, and anything less is hard to find.
The only 15w-40 I'm familiar with is Chevron Delo 400, and it is used in heavy-duty diesel engines. It has the top diesel rating of CH-4, and SJ for gas. Castrol Syntec 5w-30, by comparison, has a CF / SJ rating.
But why use 15w-40? Is this a good choice here in Hawaii, where I reckon our car never spends a night at home under 50 degrees, or sees a day over 95 degrees?
If it's a matter of corporate fuel economy be damned, this is right for our climate, great! But it sure seems a different choice than what I've been reading here.
Any informed opinions are welcome.

 

15W-40!!! Boy that's a heavy oil. I would seriously question a DEALER(!) who has no clue as to what weight oil should be used in a vehicle. Even more so, not knowing what Brand they have on hand. Sad...very sad. I believe that a 5W-30 is the appropriate weight to use in almost all new cars these days. Because of improvements in both the oil industry and automotive manufacturing, a heavy weight oil such as 15W-40 could actually be detrimental to an engines performance. The ability of a manufacturer to machine and assemble the parts of an engine to much tighter tolerances than was possible, say 15 years ago, means the oil has to flow through a much tighter passage/gap between bearings and journals. This means a heavier oil will have a more difficult time getting to all those tight places than will a lighter oil. The longer the time, the more wear occurs, especially at startup.
I would consult your owners manual concerning oil changes and recommended weights. Most manuals have a chart in them providing guidelines as to what weights are appropriate for a given temperature range. You might find that a 15W-40 to be a bit heavy for your car. That weight is usually reserved for older cars, high mileage vehicles or severe duty use.

IMHO...

Finn

 

Take a look under the hood of your WRX, the oil cap has 5-30 written on it....I believe this is the grade Subaru wants us to use unless we live in extreme climates.