Hmm, I've seen some people on this forum argue that the only oil that should go in your metro is 5W30. The owners manual recomends 5W30 for any temperatures, and 10W30 for only above 0 celcius (freezing point). Personally, I go for the 10W30 in the summer months, espeacially since we've been getting above 30 here in Edmonton lately (86 F). I have a leaky head gasket that doesn't leak nearly so bad with the 10W30 in there. I don't know what to tell you about what to use in an aging engine. I have over 200, 000 km now, and it still runs about the same as it did at 100, 000. Do stay away from things that will clean the crud off the inside of your cylinders, as I understand that it can cause your o-rings to lose their seal. I'm sure that there are others with more sage advice, so hang in there.
Jai