Flying Finn

They do but 10w-30 or 0w-40 are more viscous than 15w-50 when hot, that's how.

Feehliks

This is possible. A few general remarks as a rule of thumb:

The two numbers in f.e. "10W-30" are only two dots on a graph showing the function between temperature and viscosity. First: very low temperature, second: about 100 degr C.

Really interesting is the range beyond the second dot. The less the viscosity decreases from the first ot the second dot, the less it will further decrease in the interesting range.

So a 0W-40 is "thicker" than a 10W-40 at high temepratures, even though "40" indicates that both have the same viscosity at approx. 100 degr C. The 10W-40 loses more viscosity with evey increment in temperature.

A 10W-60 retains a high viscosity even at highest temperatures. A 20W-50 is not very recommendable. Here in Europe, 0W-30 oils are only recommended for specific engines (ecl. Porsche). Hence, I would never use a 10W-30 oil, especially not in Southern California. Here in Europe, my car has been maintenanced by Ruf the last seven years and Ruf pours in 10W-40.

Best regards,

Felix

TomF

I'm with Bijan on this one too. I switched to 0-40 on the recommendation of a friend, and the car sprouted tiny leaks in three or four places. The car had never had a leak before. I switched back to 15-50 and they all went away. The difference was dramatic. It consumed a ton of oil at 0-40 and maybe a half a quart per 2000 miles with the 15-50. I have 88K on my car and I am sure that it is a little more sensitive to what I put in it.

Feehliks

As long as your oil temperature does not reach beyond 100 degr C, in fact 15W-50 oil is thicker and much, much thicker in cold temperatures (I would not use a 15W-X oil in climate zones with harsh winters, its consistency is like honey). Therefore, Flying Finn's statement has to be corrected in this aspect. In normal driving, in particular in Seattle, you should rarely exceed 100 degr C. In other words: as long as you do not have many cold starts under harsh winter conditions your choice is alright and in fact may avoid a few leaks. My 993 has about 65.000 mls and does not have any leaks with 10W-40.

Best regards,

Felix