Re: Pennzoil
There is a law (can't remeber the name of the act right now) that says an engine manufacturer cannot deny waranty based on oil brand unless they supply the specified brand at no charge.
They can deny warranty based on a type of oil. IE - if you have a motor that is specified to run on TCW III and you were using TCW oil or TCW II (if you could find it), or 10W-40, yes they can deny oil related powerhead failures once they determine that you weren't using TCW III or higher rated oil.
It's been a while and yes, things change but when we did our oil testing, mixing brands of TCW III oils did not produce any negative results. We even mixed TCW III and synthetic blends and full synthetics. While we did not have any negative issues, we'd still recommend that you stick with the same type, whatever you choose.
We bought a boat a couple of years ago that has a 1990 200 V6 on it. The guy had owned it for about 10 years and always used what I consider to be a "not-so-good" brand oil although it was TCW III rated. The oil tank was about 1/2 full and I used the boat a couple times and when I added oil I added Pennzoil Blend. Which is what I've always used since our oil tests in my carb'd recreational motors.
Anyway, the only difference I saw was that it doesn't smoke anymore. I suspect he was going thru plugs pretty often because there were new ones in it when we got it. I haven't changed plugs yet.
Remember, TCW III is a rating. To make that rating the oil additive package has to meet a certian standard. There's nothing saying that one brand TCW III oil can't have a better additive package, it just says it can't have less and still maintain a TCW III rating. Therefore it's possible that some brands of TCW III oil could be better than others.