987 Oil Changes

[Ronzi]

This is an uninformed personal opinion, but I don't think oil loses its lubricating properties over time as much as it is that the oil becomes contaminated.
The size of the oil reservoir could also be a factor, i.e. 9+ quarts of the stuff, so the contaminants get dispersed throughout a larger quantity of oil. Plus, maybe the filtration systems are better today, also leading to longer change intervals.
Nevertheless, the oil change interval on my Boxster was about 6k the first go-round and will probably be about the same this next time. The one year interval will arrive long before the 15k miles.
The '81 911SC I had before the Box usually had the oil changed at a one year interval, and the mileage varied between a low of about 800 miles (!) to a high of maybe 7,000, and the Porsche-trained wrench I took it to grumped about that one.
The night after I bought that SC I was looking through the maintenance documents, and ran across one oil change bill with the notation in large block letters "MUST CHECK OIL REGULARLY. OIL 7 QUARTS LOW!" An oil change on that car took about 11 quarts, plus maybe another couple of quarts in the oil cooler and the lines running up to the loop cooler in the front fender, so the total system had maybe 13 qts. or so.
I suffered a serious case of buyer's remorse after seeing that bill, plus another one that noted a 6qt low condition at another change. However, I owned the car for 7 years and the engine was none the worse for it's low-oil experience. It is hard to hurt those 3-liter motors, and there is a benefit to a large oil capacity and a dry sump system.

[eslai]

Yeah, all the info here jibes well with what I know about oil. In my Eclipse, since it's turbocharged there's a lot of blow-by in the cylinders. The oil fouls extremely fast with carbon particles and gas. You can pull out the dipstick after 1500 miles, take a whiff and smell gasoline.

I change my oil on the Eclipse every 3000 miles. When I drain the Mobil 1, it comes out pitch black. If I do the same on my sister's Miata at 5000 miles, you can still see through the oil.

So, seeing as how this is both naturally aspirated and dry-sumped, I expect the oil to last longer. I've also seen many oil studied touting 20,000-mile oil changes but it makes me nervous. The dealership told me to use the electronic gauge and when it dips to the low mark, add a quart.

Oh and Randall, the dealer told me that due to natural expansion of the oil when the engine is hot, versus when it's cold, you'll get different readings on the oil gauge. He said I should just check the oil level when the car is cold and go by that.

Thanks for the warm welcome, guys.