Plenty of fault to go around on this. Yep, aggressive maintenance, with synthetics, will usually keep the Toyota sludge monster at bay. I guess the reason that Toyota modified the engine design was so it could better withstand owner neglect??? Of course, it certainly wasn't because there was anything wrong with the engine in the first place. . .
Congratulations!!! That's an absurdly clean engine, and you're doing fine. Don't worry about that fleck over on the side of the cover. This is also about how my V-6 looked when its front cover came off at about 25k for the seal repair. Double good for you if it's a 5S-FE (is it?). BTW, for the diehard syn doubters out there, this is also what my 88 Civic looked like when it was ten years old w/ 150k miles, having lived on nothing but M1 since 2k miles. Zero sludge, zero varnish.
That's the site that the crusading, hysterical 5th grade Virginia schoolteacher, Charlene Blake is involved with. She's been banned from several sites (such as Edmund's) and challenges her detractors to prove negatives. (i.e. 'prove that these engines will NOT sludge up')
The issue is not the particular vis, IMO, rather it's thermal stability, which means for the most part, synthetic. Again, this isn't a blind "anti-dino" statement. Having been a BITOG member for all of a month and a half, I'm now much more comfortable that a modern dino is fine for those engines that have not shown a tendency to get unduly hot. In these engines though (or any other with a rep for otherwise unprovoked sludging), I'd say use a syn of (or close to) the recommended grade.
As I understand it, no. Heavier oil has more resistance to flow. Flow from thinner oil improves cooling. Of course there is a practical limit on how low you can go, so I wouldn't go below a 30W. Redline 5W-20 might work well, but haven't tried and won't without seeing some results first. You're working with two fundamentally conflicting engineering goals: Low viscosity and low volatility at the same time.