Im liking ST 5W-20 for your car. You could use an (0)(5)W-30, but make sure you DONT use GC, it seems heavy for your app.
5W will flow great, just like a 0W almost! And its fine that it is ST conventional.
1) 4k oci is fine, you could go 5k or MAYBE 6k
2) dont fret that its not a synthetic, anyone who says "5W-20 ST doesnt flow well" needs their head examined. We tend to obsess about that stuff here on BITOG but trust me, your 5W-20 SUPERTECH will be fine. Cost effective, too.
Naturally, that is not the ONLY oil you can use, rather a suggestion. M1 gets pricey when compared to ST, unless they have some sale i dont know about imho. (btw, Mobil makes Supertech. More than likely. In 5W-20 its a virtual guarantee.) Id choose the ST as its "only a Sentra" and ST is not bad for your engine, no way no how. (Much bigger and more powerful engines use ST just fine.)
(CATERHAM?)