[The00Dustin]

I believe the manual (wherein I know you have missing pages, possibly including these) indicates that you should always use 10w30, though some people will disagree for various reasons. In my 1999 Park Avenue Ultra, the manual says when it is cold enough (not to exceed 20°F), 5w30 should be used. I don't believe my 1993 Park Avenue manual said that, and I believe the 3800 Series II started in 1996, so if my 1993 didn't, your 1995 probably wouldn't.

Regardless, FYI, w means weight, and 10w30 means the oil behaves like a 10w oil when cold, but at 30w oil when hot. This makes it easier to start your engine cold. Some people feel that it would be better for the engine to use 10w40 or 10w50 and say 10w30 was only used to get better mileage because of EPA requirements (others figure 10w30 is part of planned obsolescence, wear out the engine faster).

I don't really know what the benefits of the high mileage stuff are supposed to be, so I can't tell you much on that. In general, I believe regular (mineral) oil breaks down quicker than its synthetic counterpart, though I have heard that regular oil is much better than it used to be.

Regarding what oil is best, you will get a lot of opinions from different people, and you will hear things, like for instance, "most mechanics use Valvoline in their own vehicles," which may or may not be true. I know a lot of people seem to swear by Mobil1 where I live, and I know some newer GM vehicles specifically say only to use Mobil1 on the cap. I figure GM has some deal with Mobil now, and I doubt they could legally void your warranty for using other oil in those vehicles, but I really don't know if either of those theories is true.

You will also hear "rules" like "once you use a synthetic in an engine, you should never use regular oil in it again," or "you should always use the same brand." I don't know how much substance there is to such rules, but my guess is little to none, though I am sure some people would swear they matter and be able to provide reasons, legitimate or otherwise, that certainly seem to make sense.

I hear "most mechanics change their oil every 3,000 miles even if the manual says 5,000-7,000 or there is an indicator," and I have even heard this from people who worked in car shops changing oil and spoke of the things they had seen when an oil change had 10,000 miles on it (without indicating whether it had been in the car for 3 months or 3 years).

In my 1999 Park Avenue Ultra, I use Valvoline SynPower 10w30, that is a full synthetic (high mileage is probably a blend, but I don't know anything about it), and I change it every 6 months / 5,000 miles (I don't think I've ever made it to 6 months without 5,000 miles). When the car starts losing a full quart in less than 5,000 miles, (it has 165,000 miles on it and didn't leak any until recently, possibly because it was overfilled), I will stop using the synthetic and just use cheap stuff (I have heard it is thicker, but not as slick, but regardless, it is cheaper), but all of that is just based on arbitrary decisions I have made.