Which viscosity grade Synthetic oil do you use, and WHY???...

I have run out of my current stock of bulk Synthetic Oil, it's time to pony up and buy a couple more cases.
My question is, What do you all use and why.
Viscosity grades are 0W, 5W, 10W, 15W, 20W, 25W, 20, 30, 40.
I am thinking of going with the 0w-20 Mobile One.

I live in So Cal, so it's never drops below 40 degrees, with highs sometimes up to 115 degrees. I only use 3.5 Qts, change every 5k, warrenty thing, and I would like your thoughts!

Come on folks, pony up. lets have your thoughts on this subject!
Thanks, Pat KK6PD

I use 5W30.

Given your climate I don't think there's any point to using an Xw20 oil, stick with 5w30 (or 0w30 if you prefer).

Long Anwer:

0W-30 will get you some small fuel economy improvement over 5W-30 during engine warmup. If you make a lot of short trips or are just a mileage fanatic that is something to consider. The improvement vs 5W-30 might not show up in your normal mileage calculations unless you are a very consistent driver tank to tank, but there will be some small improvement.

If one of your goals is letter of the law warranty compliance and they don't allow xW-20 for your Prius, be aware that the real viscosity of xW-30 (or any 30) at 212F varies from 9.3 to 12.5 cSt. You can find the nominal actual viscosity of the oil at 212F operating temperature in most oil companies spec sheets. The lower the viscosity, the better the economy, although the difference in economy might not be real obvious in everyday driving.

Short Answer: Any good 0W-30 synthetic or a 0W-20 if Toyota allows it or if you want to take the very small warranty risk and 0W-20 is a proven good viscosity for Gen IIIs in hot climates

I use 0w-20 in my 2008 gen2 in the UK with 10k mile change interval. It is listed on the Toyota TGMO chart as being suitable for the gen2 and at 80k miles I do not need to top up between oil changes.

Hi Pat,

I use 5W-30 synthetic year-round here in southern AZ.

While I lived in south OC, I tried Toyota 0W-20 on my 2004 a few years ago during the winter months, as a very gracious Yahoo Prius group member sent me 4 qts for evaluation.

The good news is that mpg appeared to improve 5%.

The bad news is that my 2004's engine started to consume oil, which continued after I switched back to 5W-30.

In your area, I would suggest using 5W-30 in the summer (for increased protection in the hot weather) and 0W-20 in the winter if you are more interested in improving mpg and less concerned about provoking engine oil consumption.

0W-20 synthetic is recommended by Toyota for NHW20 in Japan, however I don't think that the summer weather there is generally as hot as we can find in the US Southwest.

i thought we was supposed to use 0w-20 whats all this ruckus ?

This is true for model year 2010 and newer, as that model has a different engine designed for lower viscosity engine oil.

0W20 Toyota synthetic oil and toyota filter.

I use 0w20 Mobil 1, because I haven't found Mobil 1 in 0w10 yet.

I just put in Mobil-1 0W-30 AFE (advanced fuel economy) for winter. Label states that it exceeds 5W-30 warranty requirements. Thats going to get me to 105K miles and then I am going with the extended performance 5W-30 to extend my drain interval. I think that gets me to next winter for another 0W-30 refill. So 2 refills a year going ~18000 miles/year.

First off, keep the comments coming, I had a idea what to expect, and it seems I have been on the right track!
This intrigues me, Mobil-1 0W-30 AFE (advanced fuel economy), as I have not heard of it before. I will be doing some reading.
Patrick, your comment on 0w-20 and burning oil is something I would not have expected, but I am glad you pointed that out.
XS650, that is kind of the route I am leaning towards. Sadly, "Grill Blocking" which I swore to try this winter I do not think would not work out well. 45º when I go to work at 5 AM, but usually 70º+ when I leave in the early afternoon. At that temp, I figure that Blocking would be a bad thing!

Good comments one and all, keep them coming, Thanks!

Someone with a Gen3 has been doing some careful blocked grill record keeping on his SF Bay Area commute. It's not a whole world different climate than yours, maybe about 10 degrees cooler. I can't remember his user name but he got some impressive results for a mild climate.

Actually I have less than 25K miles on my car and the first two oil changes were done at dealership at no cost to me. (They claim) I am thinking Mobil 1- 0W20 when the time comes.(30K) I have been under the car and located the filter and drain plug and found out about the safety clip on the filter element cap. Much of my work history has been electrical and mechanical maintenance so I am fascinated by the Prius technology and I have learned a great deal since I purchased the car. Prius chat is a wonderful source of information and I am extremely grateful to you all.

5W30 year-round in Dallas, because that's what Toyota recommends; I assume that the designers of the engine know more about it than I ever will.

They do, too bad we don't know what the designers of the engine would use in their own car in our individual climates and driving conditions. If we did, then we would have the best advice on which viscosity to use instead of the one size fits all owners manual.

The owners manual recommendation is a compromise. If an oil is the ideal oil for East Snowshoe Montana, it isn't the ideal oil for Gates Of Hell Arizona.

That's over near Yuma.

So you advise using a less viscous oil in a hotter-than-average climate? Why?

I didn't advise or imply that.

FWIW, I use Mobil 1 EP 5W-30 and a Microgreen Oil Filter MG 301-1, and change both the oil and the filter every 10k miles.

I figure that Mobil knows what they're doing, EP is a 15k mile oil, the Microgreen filter filters down to 2 microns (it's like having an oil filter with a built-in bypass filter).

(I don't buy into Microgreen's idea of changing only the filter at 10k intervals, and changing the oil at 30k intervals - I'm just using it as a really good filter).

At about 101k miles, I'm burning maybe 1/3 of a quart every 10k miles, so the engine's pretty tight.