5W-30 Valvoline SynPower (full synthetic) and Mahle filter since I picked this car up at 89k in '11. Just cleared 175k last week.
I think what helped as much is the valvoline oil. Valvoline really agrees with my motor. Not sure why I have trouble with some of the other blends, but it's really quiet with the ashland add-pack.
Valvoline VR1 non synthetic. It is one of the ZDDP oils. Reasonably priced and the Z seems to like it.
[Valvoline SynPower](http://www.valvoline.com/products/brands/synpower/full-synthetic-motor-oil/5) is what I have run since I purchased my 2012 Subaru Legacy.
You don't need to add any more additives to modern day oils. You said you will be using Mazda 0W-20 oil? This is one of the best 0W-20 oils on the market. It is made by Idemitsu for Mazda. It has very high Viscosity Index of 221. Most 0W-20 oils are around 170. This means it is one of the lightest 0W-20 at start up. This is what you want in the winter.
Anyway, when I had my Passat 1.8T and sludge issues were the big fad, my oil of choice was Valvoline Synpower 5w40. It met 502 and was not a synthetic blend but rather a real synthetic.
I use valvoline because it's common and it does the job. My auto mechanics instructor also recommended it because he said most of the engines he'd taken apart that had run valvoline looked pretty crud free.
Strictly anecdotal, but at 56,000 I switched from the usual suspect Castrol to Valvoline Synpower. Engine starts more smoothly and observed mileage on highway has gone from approx 30 to approx 34 mpg.
I use 20/50 Valvoline year round....
I was recently obsessed with oil in my 01 1.8T. I was at http://www.bobistheoilguy.com alot and determined that Valvoline Synpower 5w40 was the way to go based on oil analysis and availability. Castrol Syntec 5w40 appeared to degrade faster and the viscosity wasn't near as good as the Sypower after only 3k miles on it. IMHO Synpower outperforms Syntec in our motors based on actual facts and numbers from the oil analysis.
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