The enemy of oil is thermal breakdown, Amsoil has properties (additives) that resist thermal breakdown by actually getting thicker (increasing viscosity) over repeated heating/cooling cycles.
While this is great for longevity, it can cause major problems in an engine that specifically is designed for a certain oil viscosity, as cylinder de-activation (MDS) engines are.

There are many documented independent tests to confirm this and I'll give an excerpt from one:

By rights it should be the absolute last sample, but we're going to spot Amsoil one more after this. At 13,000 miles, Amsoil continues to hold steady -- clearly, Amsoil can last much longer than we initially thought possible. That is provided, of course, that you have no problem with the oil thickening way out of grade (many people don't, which is why we continued the test despite our own reservations about it now being a solid 40-weight oil). Well, one more sample to go, and then we'll enter the next flush period, which should settle whether the oil is contributing to the fuel economy drop.

This is it. Amsoil is done! The oil exceeded a year in service, with 14,000 miles on the ticker, and no filter change! Well, if the year hadn't ended, we would have had to change the filter now -- it finally reached our insolubles cap, 2,000 miles after Mobil 1. The main thing that stands out on this, our final Amsoil sample, is the ridiculous viscosity. This 5W30 oil has now thickened out to a 15W40 -- argue whether it matters if you like, but we believe engine builders spec an oil for a reason, and this oil is far, far thicker now than intended for the LS1.

 

Well, for what it's worth, I do have MDS turned off. And I only put about 8k-9k miles on my truck per year, so there is never more than 4k-4.5k miles on the oil filter. I also have my oil analyzed by Blackstone Laboratories once per year and my truck consistently gets a "clean bill of health". To each his own when it comes to oil I guess.

 

For those who don't like Amsoil, Royal Purple & Red Line one heck of a lot better then Mobil 1.

 

here's another fwiw..

ALL oil, regardless of how well constructed, will become contaminated with ash as it ages. The properties of the oil survive, depending on the quality, but the addition of ash and other contaminates becomes THE concern..

you can use the best oil on the planet, but if you don't change it regularly, it doesn't matter how good IT is or isn't, because the contaminants that WILL infuse it becomes the 'problem'.

you can use external filtration and increase volume, but you're not stopping the ash from getting in it..

Dino oil is much better than it was even a decade ago.. It is likely better today than synthetic oil was at the turn of the century.. There is not a thing wrong with using it- SO LONG- as you change it..

Filters are just, if not more, important. Using a paper filter is asking for trouble. Using a paper filter for more than 3k miles is really pushing the envelope. For the love of God, get a good filter!!

all that being said, I use synthetic too.. it's quieter, and it seems to maintain the 'something like honey' look longer than decomposed lizards.. I change the stuff every 3-3.5k miles, though.. I like my truck.. I'm going to change the oil.. often.. because that ensures the oil is doing what oil is 'sposed to do, and I won't get a surprise when I see pressure drop whilst driving down the highway because a paper filter said "eff it, I'm done", or see chunks flopping out of the pan when I pull the plug..

 

I should have expanded on my comment, but have a few times previously. I don't have a problem with Amsoil, it's a quality product and I really don't have an issue with it in an MDS engine - provided it's changed around 6k miles. All tests I've seen don't show a significant thickening of the product until after about 8k miles. It's the extended use of the oil that I caution against and given the cost and the fact that ANY quality synthetic will perform perfectly for 6k miles, I just don't see a benefit of using it.

I've had Valvoline synthetic tested at 6k and the reports have come back perfect as well. I catch it on sale for around $26 for a 5 qt. jug with a free $12 K&N filter plus they pro-rate the two odd quarts at the sale price per jug. A lot cheaper than running Amsoil for 6k miles (or RP which I have run for that matter)...

 

Great info gentlemen. Thanks for expounding.

 

I switched to Mopar filters as well as soon as they started being available on the box stores shelves. My local dealer went under so I don't have them anymore as a resource for Mopar parts. The only other options, where I live are Fram and a couple misc other ones, depending on where you go. Now and then, I get my tire shop to change my oil when I take it in for a rotation and they use Napa Gold filters.

 

I got my Mopar filter from Walmart.

 

considering the switch as well... just wondering what weight ya'll are using... keep in mind I am from the frozen north (canada haha) and i do tow a 21 ft travel trailer... summer time does get quite warm around here, high 90's is usual sumertime heat

 

I don't know if it's the case anymore or not, but as of about four years ago the Mopar branded filters at the dealership were made by Champion Labs (makers of RP, K&N, and some other real good filters). However, the commercial ones at retail stores were made by Fram - no good...