I just thought I would add one more thing before this becomes a dead thread...
In my first post, I said that whilst oily intake deposits don't particularly look good, they aren't anything like as harmful as the oil that makes it through the intake and into the cylinders to be burnt. Burnt oil makes for dirty pistons, stuck rings and all sorts of bad stuff you don't want to experience in real life.
Well I stumbled upon some nice, illustrative piston photographs in the blurb for Eneos's Sustina engine oils. Please have a look at these by clicking on the link below...
http://www.eneos.us/products/sustina-enhanced-performance-motor-oil/
These pictures are of pistons taken from the industry standard Sequence IIIG test. The test uses a 1996 GM V6 engine which runs for 100 hours continuous at 3600 rpm and with the engine bulk oil temperature kept at the relatively high temperature of 150°C.
Three pistons are shown. The bottom one is very dirty representing GF-4 quality. The middle one is still dirty but marginally better and supposedly represents GF-5 oil quality. The top piston is very clean and represents what you purportedly see with Eneos's synthetic oil. In terms of Weighted Piston Deposits (where 0 is God awful and 10 is pristine), I'd guess you are looking at ratings of 3.5, 4.0 and about 7.5.
Now you're going to have to trust me on this but what I would say you're actually looking at is...
a) a Group II oil with a Noack of 14 - 15%
b) a Group II/II+ oil with a Noack of 12 - 14%
c) a Group III/IV oil with a Noack of below 10%
This should give you some idea of how Noack potentially impacts on piston cleanliness and why it's a good idea to go with a low Noack oil.
As an aside, you might ask yourself the question why the API & ILSAC ever got it into their stupid heads that the bottom & middle pistons represented an 'acceptable' level of oil performance for GF-4 & GF-5? (especially when you realise that this extreme level of deposit was laid down over just one OCI!) You might also ask, if this is what high Noack oil does to pistons, why the API & ILSAC haven't taken steps to further reduce the Noack spec from 15%? After all, European PCMOs have limited Noack to 13% max for years and dexos oils have also moved to this limit. From what I've read, the 15% Noack max limit will carry over into GF-6. It's insane!