I'm reviving this sort of old thread here with some updates about the car, for anyone interested.
I switched to Valvoline Maxlife High Mileage Synblend 5w30 earlier in about April. The car runs way better with it compared to Supertech 10w40. Using a 94 Jetta NAPA Gold filter, as I have a stockpile of about 5 and they fit in the threads. I had some oil loss during the first thousand miles, about half a quart, now it seems to be using none at another 500 miles. The car just felt immensely smoother with the Valvoline Maxlife 5w30, though. Really was a completely night and day difference in smoothness and ability to rev up.
Bad news is, I do have an oil light staying on for about 2 seconds after startup, instead of immediately turning off. I think it's some combination of the oil filter getting clogged with sludge, the oil being a bit thin with it being so hot out and me taking longer trips/constantly driving it throughout the day now compared to in winter, since I have to drive for other family members now with the Corona situation and health issues. So my plan is to change it ASAP, and I bought two gallons of the NAPA Valvoline 15w40 during March when it was $10 a gallon with Corona craziness thinking the world would end and I'd be king with two gallons of 15w40. So I will likely change it with this 15w40 as I have it available and it's hot out. I'll probably still use the Maxlife 5w30 in fall/winter in it for sure, though. Hopefully the HDEO cleans sludge out well, too.
For other good automotive fluid related news, I switched it to Valvoline Maxlife ATF and that made a world of difference in how smooth it shifted and just ran in general. I had Supertech high mileage ATF in there, with Lucas Stop Slip (trans was iffy feeling and jerky) and then Seafoam Transtune put in after to see if it made it better, which it did, so I think thicker fluid made it worse? Anyway, it shifts close to perfect now with Valvoline Maxlife ATF. REALLY impressed with it. Overall super impressed with Maxlife oil and ATF. A guy who works on BMWs on Youtube is a giant fan of the Maxlife ATF in those boxes, so I figured why not for $18 at Wal-Mart?
Lastly, the biggest running related thing I did that seemed to help things out immensely as far as fuel economy (from what I can tell, but inconclusive) and power in general was spark plugs. I had NGK V-Powers in a 5 heat range in the car, I switched to a 7 heat range and it just made the car run absolutely fantastic with loads of power, quieter and smoother idle, and better hill climbing ability and some ability even to "lug" the engine like I could in a manual car up hills, which it would never do before. My sort of guess is besides maybe my timing being a bit too far advanced (when I got it, it was too retarded by a significant amount...) is that there's a lot of carbon on the pistons and head, so running a colder plug prevents me from detonating and the car can run stichiometric. It used to smell like gas everywhere it drove, and now with the colder plugs it's solved it and it no longer smells like that out of the tailpipe.
So that's an update on the car. Unsure of the rest of the engine but I got about 170PSI out of all the cylinders when the engine is hot, testing out compression. So after 265K, it's still within the low end of a healthy compression range (164-190) and not something awful like 120-130 like I was initially thinking, and that's with 5w30 and not 15w40/etc to try to eek out more compression.