Use wynnes when engine is hot, yes using it in smaller amounts over over a few oil changes will work better, remember the micra only has around 3 litres of oil, the wynns flush can treat cars with 5-6 litres of oil. I have taken off cam covers before and after 15 min treatment of wynns in a corolla and it didn't clean up the sludge (this engine very heavily sludged) but to be more effective in 15 minutes it would need to be so stong it would dissolve the oil seals. Don't expect miracles but as a oil change service cleanup it is fine.
Approx thinest oil first:
0w20
5w20
0w30
5w30
0w40 - 10w30 similar depending on manufacturer
5w40
10w40
15w40
10w40 is slightly thinner than 15w40, don't use 15w40 unless the car has done way over 100,000 miles and has probelms. 15w40 type oil are usually used in old minis metros and other old fashioned cars. On a sound modern Nissan 16v engine runs best on 30 weight oil, but in this country 10w40 is more available, 5w30 is common now cos it's used a lot in ford zetecs.
On your next oil change get one of these filters. All motor factors and parts stores deal with Blueprint parts, these are for japanese imports and really good quality. Ask for a ADN12110 they are usually around £5-6 but are way better than champion and nissan filters. When I put a Nissan filter it was made in spain and seemed poor, i noticed a lslight tick on startup for 1-2 seconds, with the Blueprint filter it had gone it's cos of the orange silicone anti drain valve this is essential on our CG chain driven engines.