Engine oil Castrol or IDEMITSU

Castrol GTX 5W-30 with an STP filter. NY area. 178k miles with no HLA tick so far. Had some 10W-30 in it before during the winter and the car didn't like it too much, took over 3-5 tries cranking to get it started.

In my gen 3, I've used Castrol OE Professional 5w40 (the Castrol in the gold bottle that the dealer sells - oil has a green tint to it), both when I did the 5k mile change and when the dealer did the 10k change.

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.

Used Castrol 0W-30 in the past with the GTI and GLI. Pretty decent oil analysis numbers when tested.

I prefer Castrol 20w50 in my 1976 CB550, at least in the summer. Non-Synthetic

I'm using Castrol Syntec Euro formula which is a 0-30 oil. I've been doing oil analysis with every oil change since I got the car. This change was a short interval with just 1400 miles on the oil between changes. Right in the middle was a track day where I did about 130 miles on track, six 20 minute sessions. I think I was seeing peak oil temps in the 240s with very quick recovery back down below 220 during cooldown.

Castrol A747. The only failure I've ever had was down to incorrect assemble. Smells lovely too!

I use Castrol Syntec 10w-40 in all of my VR6's and change it around 3000-4000 miles with oil filter (Meyle, Hengst or Topran) I use it in my MK4 and even in my old ass MK3 and never had any problems whatsoever. My MK3 VR6 that a sold a few months back, a.k.a. The Silver Bullet, has over 150K miles on the odometer and still counting and she runs like a beast. The lifters are ticking a bit loud but that's common for any high mileage VR6 engine.

Vw recommends Castrol I wouldn't use anything else

Castrol Syntec 5w40 is stout enough for the 10,000 mile oil change.