Valve guides do wear out on 4V per cylinder heads on VW/ Audi and do lead to oil leaking down valve stems but usually it's not much of a problem. When I had my AHC engine apart at 124k miles they were a bit worn but not too bad. Replacing valve guides with bronze ones would have cost me over £1k so I was just going to put heads back as they were because there was not much oil usage, then I found pair of heads that have done only 2k miles since supplied by Audi for £400 so I put them on my engine. I was a bit confused about my oil usage because sometimes it seemed like it lost a lot of oil with very little miles, then again like it gained oil after some more use. What I discovered was that many modern oils (I use Quantum Longlife III) expand a lot when warm so the reading between warm and cold engine varies a lot. The correct way to check oil level is to get engine to 80-90C, switch off and check level after 5-10m to let the oil run down. I found that on warm engine it would show max level, but if I left it overnight it would be just over min level.
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PO said he used 5 -30 synthetic. I've been topping up with 10-40
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I never recommend mixing oils, it's not that bad if it is the same brand but otherwise there are so many oils that there is no way they have all been tested for cross compatibility to make sure additives does not react and cause any undesirable effects when mixed.
I am not a fan of stop leak additives, some may work, some work for a short term or does not work at all. Basically they contain solvents that soak into rubber and make it swell up but can make rubber too soft and brake apart if there is too much of them or if it is exposed for too long. It all depends on materials used in car, condition, additive, concentration and the time used. Think about dried, hard bread, you could use some water steam to get it soft again but if you dunk it in the water it will fall apart.
My suggestion would be- check for leaks and eliminate where possible ( and cost effective), head cover gaskets and camshaft seals are known to leak as ids the oil cooler. Short trips can use more oil than long ones and if you do a lot of short trips it could look like there is no drop in oil level because engine uses oil but replaces lost oil with unburned fuel, then you do a long trip and suddenly oil level drops because that unburned fuel evaporates from oil when engine gets to temp and stays there for a long time.
ABZ engine is fairly old and I don't think it will fully benefit from 5W30 oil, you could use thicker and cheaper Quantum Synta 10W40 to reduce oil usage and save money.