Here is one of the best most plausible answers to oil 'thickening' from a oil forum. I think a perusal of the UOA section on this site could show you some situations where thickening and thinning of the operational viscosity of oil may occur.

Thickening - as mentioned occurs due to oxidation and thermal stressing of the oil. It is engine specific, climate specific, and so the only way to know is to use your own UOA trend.

Thinning - also as mentioned can result from sheering, or in some cases from fuel dilution. Again this is engine specific and requires UOA.

Regardless of what oil you are using, if you are going to extend out your oil life, then you need to do UOA for several changes to see how that oil is responding to your specific circumstance.

I can't recall hearing any one oil being bad-mouthed for extended service due to oxidation. However, I do know that in my wife's car we can't do more than 20K km (12500 miles) on BMW Private Label Castrol before oxidation starts becoming an issue. (I have proven this through extensive oil analysis on her car). This despite the dealership recommending 24000-30000 km every time I take it to be changed.

 

Some very good information in your post hounddogg. I can't speak for others, but I certainly didn't bash Amsoil. My point was that when you recommend 25,000 mile oil changes as Amsoil does there is a very good chance the oil will thicken out of grade. Hence the recommendation for UOA if you're going to go 25,000 miles.

Your experience with your wife's car and BMW private label Castrol is interesting and similiar to my own experience with my 740iL. I don't use BMW's oil, I primarily used Castrol 0w30 ("German Castrol") which is a PAO synthetic. Despite the oil change monitor showing I could go 25 - 30,000 km I generally changed at 15,000 km or less. Interestingly, years ago I purchased a 600 page Bentley Service Manual specific to the 7-Series and in one section it said "for maximum engine life, change oil and filter at half the interval recommend by the oil life monitor". Don't get me started on BMW's "lifetime" transmission fluid...

 

That BMW is not mine. It was a reply from a forum member on a oil site. That was a reply to my question about oil 'thickening'

 

I used Amsoil in all my vehicles because I only pay $4.70 a quart. I even run it in my lawnmower and garden tractor. I don't know if it is better or not. I just like the price and the convenience of it being sent to my front door.

 

I live in canada and have used amsoil for years. with the temps we have here in the winter you would recognize thickening on a cold day immediately. i had it in a turbo 4cyl subaru for about 20,000 mi and sent it away for analysis and it still was perfectly fine to continue use and had a higher tbn than dino oils. my harley doesn't clunk as bad going into gear with it in also. the difference in the oils is how long you are keeping them in.if you are only changing at 10k, the top of the line 0w-20 isn't necessary.

 

I was at Walmart today and all their top line synthetics were above $7.00 a quart. I think Purple Power was greater than $8.00. In my opinion all the synthetics on the market are just fine. I have read in many places that all but Amsoil start with a base of dino. Another thing for thought is that Amsoil was the first to make synthetics, I believe in the early 1970s. Just food for thought.

 

WRONG!!!!

Amsoil was not the first synthetic company but some use that line to boost their image. Synthetic was actually developed by the Germans in WW2 and actually produced by Motul and Synsol. Additionally Chevron produced the first commercially available synthetic in the 60's.

I'm not here to bash Amsoil but tell me guys, if that stuff is sooooo great, why isn't it the choice of factory fill synthetics for vehicle manufacturers? Snake oil.

 

Its the unicorn tears, it prevents them from performing industry standard tests

 

it's just like shocks or tires, they don't put the best on a new vehicle, they buy the cheapest one so they can make more money.

 

It seems that factory fills are to promote their own. Again, as I said, price, price and price. Why pay more for the same stuff when you can get an equal cheaper? Snake oil! I don't think so. Advertising hype! I think so. I don't think Amsoil is any better than Mobil 1, etc. If I can get a quart of Mobil 1 for a lesser price, then that is what I will get. I have even used Walmart synthetic in my vehicles. Gee! To Walmart, all the top brands are snake oil.