We have a 2005 Forester non-turbo which calls for 5w-30 motoroil. We have been using 10w-30 weight since we live in So Calif. Will this cause any engine damage over the long-term.

why would you use a different weight oil than the owners manual recomends?

Naa. Down there it will actually protect your motor a bit better for the conditions you get. The 5 part would be great for cold strating in cold weather and should help get a tiny bit better mileage and emissions.

I am near SF and I run 15-50 in my Forester but it has a lot of miles.

The owner's manual probably also assumes you are doing mostly stop and go driving where the oil doesn't really get up to full temp very often. If you do alot of highway instead, a heavier weight is probably good.

Depending on if you are using synthetic or regular oil the viscosity may seem a little different.I have found that 10w30 synthetic flows like 5w30 regular.I have no proof to back it up I have just found that I really like sythetics alot better.

If you check Bobishtheoilguy I believe the chart indicates that synthetics do tend to be a bit lighter.

Perfectly fine to run 10W/30 in sunny California, which is no different from 5W/30 at operation temperature and better at retaining its visocosity.

At operating temperature there is absolutely no difference in the viscosity of 5W30 or 10W30 motor oil beyond the normal brand variances. The first number folowed by a W is the cold viscosity so 5W30 flows better at startup than 10W30. The second number is the viscosity indicator at 100 degrees centigrade (celcius?) and the oil must meet the viscosity range specified by the API to be labeled as such.

 

Short answer, in your climate it doesn't matter much.

crane, that is NOT true.

 

The xW-xx numbers are appoximations that sum up a range of oil viscosities. You need to read the tech data sheets if you want to be serious about oil thickness.

 

For example: Mobil 1 5W-30 is viscosity-wise almost exactly the same as Shell Ultra 5W-40.

 

Top and bottom of the ranges...

why would you use a different weight oil than the owners manual recomends?

Well, let's take a look: (per '02 Impreza manual) "Engine oil viscosity affects fuel economy. Oils of lower viscosity provide better fuel economy. HOWEVER, IN HOT WEATHER, OIL OF HIGHER VISCOSITY IS REQUIRED TO PROPERLY LUBRICATE THE ENGINE"

 

My manual has a little temperature graph indicating that 5W-30 is OK up to 100*F. The chart also has a second arrow indicating that above 0*F 10W-30 & 10W-40 are applicable.

 

Then it reads: "If the vehicle is used in desert areas, in areas with very high temperatures, or used for heavy-duty applications such as towing a trailer, use of oil with the following grade and viscosities is recommended: SAE 30, 40, 10W-50, 20W-40, 20W-50"

 

It wasn't that long ago when all owners manuals correlated viscosity with vehicle useage and ambient temperature, but the trend is toward manuals with a dumbed-down one-size-fits-all viscosity recommendation. It's to Subaru's credit that they still put the information in the manual for people that are willing to read it.

yep, this used to be stated on most info and now Subaru is one of the few. My BMW manual also makes this pretty clear. I guess they know folks will drive them all over the world.

I think this guy is using the correct oil for his area since you will never see cold and you can get stuck in traffic in the summer which can be as bad as towing.

yep, this used to be stated on most info and now Subaru is one of the few. My BMW manual also makes this pretty clear. I guess they know folks will drive them all over the world.

I think this guy is using the correct oil for his area since you will never see cold and you can get stuck in traffic in the summer which can be as bad as towing.

 

This is good info but correction is needed on the "stuck in traffic in summer" part. Excessive idling and stop and go driving puts lots of demands on cooling system but doesn't increase oil temp. This is a common misconception and only people with cars equiped with oil temp gauges can attest to the fact that engine oil is lukewarm in city driving. Oil is only hot with high speed, high RPM driving and thus, racing cars have engine oil coolers.

This also why cars sold in US are recommended with 5W30 or 5W20 oils while most cars sold in Europe usually call for 5W40 or 10W40 oils.

That's interesting Friendly, I'll have to watch the BMW oil temp guage more in traffic. I'd swear it goes up when I sit there and idle, the water temp guage sure does. It also goes up when climbing the Grapevine which is more akin to the high speed high load stuff.

A grade or two in either directions isn't going to make much difference to engine life. use the Subie recommended ranges as a starting point and make a decision based on your particular situation.

 

Jack

Hey I was wondering what would happen if you mixed 5w30 with 10w30? Since I have less then a quart of 5w30 I would like to use up on my next oil change when I change over to 10w30.

Not much. The 10-30 gets a tiny bit lighter. I mix regularly on my BMW. I start out with 5-30 Mobil 1 and it takes a year for the next oil change. In that time I top up with 10-30.