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When digging back through some older posts looking for some UOA's for a recent thread I found a number of Pennzoil Ultra Euro 5w-40 UOA's that showed a tremendous amount of viscosity loss. Now, one of them had significant fuel ingress as observed via a large reduction in flash point but another, which ended at the same viscosity, did not.
http://www.bobistheoilguy.com/forums/ubb...VW_#Post3664472
and
http://www.bobistheoilguy.com/forums/ubb..._Eu#Post3632653
Now, I find this somewhat amusing as of course the 5w-40 grades are oft touted as being more stable than the 0w-40 lubricants, yet in this case we have two examples that clearly show this not to be the case. We have what must be hundreds of M1 0w-40 UOA's that don't have this type of viscosity loss, even with significant fuel dilution.
Now, there were some UOA's where viscosity loss was not present (and one from Astro14 where the viscosity actually increased, which I believe indicates oxidation and of course Quattro Pete's UOA's showed very little viscosity loss) but there are others that still ended up like this:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1176065
10.53cSt
Or this one:
http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/2253386/1
Where the 5w-40 dropped to 12.1cSt while M1 0w-40 in the same engine remained close to virgin at 13.2cSt
Now of course viscosity loss itself doesn't indicate an issue or that the product did not provide adequate protection and that's not the purpose of this thread, it is more to discuss causes of viscosity loss above and beyond fuel contamination as sometimes even the same model of engine will have very different viscosity loss than another fitted to a different vehicle.
http://www.bobistheoilguy.com/forums/ubb...VW_#Post3664472
and
http://www.bobistheoilguy.com/forums/ubb..._Eu#Post3632653
Now, I find this somewhat amusing as of course the 5w-40 grades are oft touted as being more stable than the 0w-40 lubricants, yet in this case we have two examples that clearly show this not to be the case. We have what must be hundreds of M1 0w-40 UOA's that don't have this type of viscosity loss, even with significant fuel dilution.
Now, there were some UOA's where viscosity loss was not present (and one from Astro14 where the viscosity actually increased, which I believe indicates oxidation and of course Quattro Pete's UOA's showed very little viscosity loss) but there are others that still ended up like this:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1176065
10.53cSt
Or this one:
http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/2253386/1
Where the 5w-40 dropped to 12.1cSt while M1 0w-40 in the same engine remained close to virgin at 13.2cSt
Now of course viscosity loss itself doesn't indicate an issue or that the product did not provide adequate protection and that's not the purpose of this thread, it is more to discuss causes of viscosity loss above and beyond fuel contamination as sometimes even the same model of engine will have very different viscosity loss than another fitted to a different vehicle.