Oil spec and performance ratings

[..P]

There have been some recent discussions about the appropriate oil viscosity rating for various Porsche cars. My 1990 944 S2 owners manual, for instance, calls for a 20W50 oil for outdoor temperatures that range from –10 degrees up to over 100-degrees. However, it also calls for a 20W40 and a 15W40 for the exact same outdoor temperature range. Furthermore, it only classifies those oils as “multigrade”, having to meet only very basic requirements. Then the owners manual shows a “fuel efficient oil” of 5W40, 5W50, and 10W30 being specified (by Porsche) for the same temp ratings (and way beyond, for just about "any" temperature). The “fuel efficient oil” is referring to an engineered synthetic, by the way.

One thing I would like to share with the forum, is the fact that oils of the same weight designation will vary vastly in their performance. Look at the following link for a moment and please take a look at the 20W50 oils that have been tested.
http://www.vtr.org/maintain/oil-overview.html

You’ll see Amsoil and Red Line 20W50 products (both are premium synthetics) will flow down to –38 and –49 degrees respectfully. Both of these oils also have a very high Flash rating (that rating measuring ability to withstand heat before breakdown, 400-degrees being the industry minimum), and Amsoil is at 482, with Red Line at 503 (higher is better).

Now, on the same 20W50 chart, you can find a 20W50 Valvoline oil that only flows down to –10 degrees F, and has a Flash rating of (only) 425 degrees F, just 25 degrees above the minimum acceptable. It certainly looks like Amsoil or Red Line would be best for an expensive Porsche engine. Looking at the Castrol GTX 20W50, which is a good natural oil, you’ll see it’s flow performance is only down to –15 degrees, and it has a reasonably high 440 rating for the Flash (neither of these ratings are even close to the Amsoil or Red Line products, but the oil still has the very same 20W50 rating). You'll also note that Castrol burns with a heck of a lot of left over ash.

There is a vast difference between motor oils of the very same weight designation.

Now look at Mobil-1 15W50, my favorite for the price paid, it has a cold flow waaaay down to –55 degrees, and a Flash rating way up there around 470. You’ll note the impressive performance of the 15W50 Red Line too, but you can find oils in any category that look like “pure crap in a can”, and they have the exact same weight designation as the super-high-performance products.

What this means is this: “if you only select an oil by what you read on the can, or by what the marketing department tells you, you are un-informed and basically flying at night without instruments”.

I use Mobil-1 15W50 in my 928 and two 944’s. I’ve tried the Mobil-1 10W30 in the older 944 in wintertime. I’ve also tried Amsoil 20W50 in the 928. Based on my experience, I believe the Mobil-1 15W50 is the biggest bang for the buck, and I also use it in a 1966 38’ fiberglass Chris Craft cruiser powered by twin 427 Ford motors.

You must select the oil you believe is best for your particular climate, your driving habits, and the milage and wear on your particular car. I hope this information will assist you in making an informed choice.

Sincerely, “P”