oil change??

Won't hurt anything to change to either one.

Why do you want to change? My 99 Sentra has 120K miles and I just use any of the cheapest 10W30 I can find. Don't fall for the marketing fru ha ha nonsense about high mileage oils etc. Oil is oil. Synthetic is better, but who really needs it? Actually, you should be using 10W30, not 40. I use synthetic in my wife's car so I don't have to change the oil as often. Other than that, I just use regular.

CD

Codifus quipped:

oil change intervals should *never* change just because you're using synthetic. There's three main purposes for oil (1) lubricate, (2) cool and (3) suspend/trap foreign particles.

Your engine produces the same amount of contaminants regardless of the type of oil that's in it. Letting it slosh around for several thousand more miles than the recommended oil change only allows the oil to get dirtier and provide less lubrication. Additionally, the longer the oil is in the engine, the more acids build up.

Synthetic oil is superior to dino oils in every aspect (including price), however it should never be changed at longer intervals unless the oil you're purchasing is specifically developed for longer change intervals. And if you use such longer interval oils, make sure the oil filter is ALSO made for longer intervals or you're just pushing dirty oil back through the engine.

Although there may be those that wouldn't agree, I have been told by several shops that PUSH synthetic, that to change a car from dino to syn after 100K often invites OIL LEAKS. Syn oil tends to migrate through seals and those tiny areas where dino doesn't venture, and often times it seems that older engines will develop small oil leaks when this change is made. Based on what I've been told, this is not true for an engine that started using syn early on, nor is it true for low mileage situations.

Also I note you said you were using 10/40. Unless you're driving in the desert, (and perhaps not even then) I believe the recommended oil would be

Willy

"Willy" quipped:

well...the short answer is that you're correct, the long answer is

The reason that *some* cars will leak synthetic oil is that synthetic oil molecules are uniform in size and tend to be smaller than dino oil.

Dino oil, OTOH, are not uniform. Like marbles of varying sizes, some of the molecules are larger than synthetic molecules. So, in the case of leaks, if the sealed area has a gap large enough for synthetic molecules to pass through but not the dino oil molecules, you'll get a leak.

Only in a general sense does that directly relate to the age/mileage of a car. When a car gets older, tolerances between parts and gaskets become wider, thus causing leaks...even with dino oil. However, since only a *portion* of dino oil is small enough to pass through these "cracks", the effect is not noticed (or so minimal as to not be visual except under close examination). Synthetic oil, if one molecule can pass through, ALL of the oil molecules can pass through. Hence the reason that you'll experience leaks.

Having said that, there's no reason not to switch to synthetic oil and if the leak(s) are severe, then switch back.

There's no evidence that starting with synthetic oil will guarantee that future leaks won't occur.

I agree with you. In fact, just switched my Murano with 105K to syn and have had no sign of leaks.

Willy