Oil - 15W-40 or 20W-50?
 

Hi people,

I am wondering which oil I should use for my new RX8 2009:

15W - 40 or 20W - 50?

I live in a tropical climate - 30-40 celcius on average - humidity is high 70% but not dusty - clean air generally.

I noticed that 15W - 40 is usually more expensive, so which one would be better for the engine?

And my dealership will provide free service every 5000km and they provide 20W - 50, but can (and should I) top up with the 15W - 40 (i.e. mix em up)?

Thanks.

Your dealer will provide 20w-50 and knowing yours is RX8 ? Interesting.

use 20w50

and yes, you can top it off with 15w40

I am using 20w50.

Im gonna say that you should start reading here. Personally everyone I know runs either 5w20 or 5w30...never heard anything higher used.

They gave me a 1 litre bottle of 20W 50 to start off with, so I assume come service time they'd be using the same stuff.

I'm just figuring out if I should buy 15w - 40 for my top ups instead and mix em. or perhaps 5W-20s though I haven't seen them around where I live (note - 5W-20s suitable for hot climates?)

and I've seen one (Shell) with SM rating as opposed to SL. Is that suppose to be better quality?

With your climate ? I'd stick with 20w-50 like NYCGPS said. You can mix and match if you want (fills with 20w-50 and top-up with 14w-40).

And how many engines have you gone through at this point with your 'superior' oils?

30 - 40 C is between 84 - 104 degrees F,

What do the AZ guys use for oil? their climate seems harsher then the OP's because of a lack of humidity...

5w30 seems thin for hot weather like that, but 20w50 seems a bit much. how about 10w40?

Ok...so after some reading about oils and viscocity I have some questions. The biggest one being...why on earth would anyone with an 8 want to run such a heavy oil. While I can understand that with the warmer low temp that one would want to start with a thicker oil as you don't have to protect with it dealing with temperatures lets say around -17C. But is it wise to move from 5 weight, Mazda recommended for everything as far as I know, to 20 weight? The other question which might even be more important is...how does the OMP handle the heavier oil? Doesn't one risk engine damage by going too heavy?

the recall came out for a reason. and i believe damage was already done before i even start premixing. the premix actually extend the engine life for a bit, if i never premix, my engine probably died long before then.

and dont forget , no matter which engine oil i use, it does not fix the lack of omp rate the recall calls for(stocck omp is pathetic anyway, but lets not get into that), with this engine, Im premixing since day 1.

also, my old now gone engine could be a bad build since day one, my crazy oil change probably gave it a longer life.

heavier weight engine oil will be able to protect bearings better than thinner oil. thats fact. its gay to see eccentric shaft wear down to copper at early age. i have oil temp guage installed and i constantly sees oil temp between 70 to 110 celsius. so is 5w30 even enough ? i doubt that

*Edited for some error, was posting this originally on my Xperia*

There's a thread somwhere on the site with the oil recommendation page from the Australian owner's manual. It has nine temperature range bars and at least a dozen grades of oil. Worth looking for...I'm not good enough with the search engine to find it.

Two worthwhile items on that chart are:

o 5w20, 5w30 and 5w40 are all shown as applicable for -25 deg F to 120 deg F
o 20w50 and 15w50 are shown as applicable for 0 deg F to 130 deg F

So, as long as nycgps does not go out for a drive the once per decade or so that NYC gets sub-zero weather, he's fine. He's also well prepared for global warming.

And everyone else can choose virtually any grade of oil they want.

Personally, I'd keep away from the really thick stuff since that will adversely affect cooling. But I have no expertise, and didn't even stay at a Holiday Inn Express.

Ken

Odd that they left all the 10w stuff out of that info you have there Ken. I guess the other question that I have is...the viscocity temps are measured via the actual fluid temp. So couldn't one measure the temp of the oil lets say on a dipsitck to determine what range you would need? I can't imagine the engine oil staying under 120F if the ambient temp is lets say 110F.

They've got a bunch of stuff on 10wwhatever, and also on some straight grades. I just typed in a couple that were pertinent to this thread. The page is somewhere on this site.

The temp ranges specified in that kind of chart are always ambient temperatures.

Ken

Hrmm...i guess that since viscosity is measured by liquid temp I just assumed that that chart was too. Wouldn't things like ambient humidity and air pressure play into that then? I would think that it would impact the effectiveness of the oil coolers along with engine performance.

As long as you drive more than 30 minutes per trip and your area is not dead freezing cold at all times like 10 Degree F or lower, your engine oil will see at least 75 Celsius. In 10-15 minutes of slow traffic, it will go up to 100 Celsius in no time.

starting this winter, I will not drive my 8 as much in the winter as I used to be(daily), instead I will drive my FIT (or whatever I have in the future if I sell it for something else), so I might stick with 20w50 (or lower it to 10w40, depends)

My Honda fit uses 5w30, factory calls for 5w20, but I dont buy that thin oil crap. I might even use 0/5w40. depends.

But it will see ambient when it first starts. That's when you need the bottom number.

Ken

I live in Jersey. It ranges from 0 Degrees Fahrenheit to 105ish Degrees (rounding). I use 5w~30

10w is fine for most.

According to my Mazda manual 10w 40 can handle up to 40c and 20W 50 up to 50 celsius which is why given our extreme temperature i am choosing to use 20W 50. I plan to buy 10W 40 as top-ups though.