Which weight oil?

My '92 Metro repair manual says to use 5W-30 oil for all conditions, or 10W-30 in conditions above freezing. It also warns agains using any of the heavier weight oils. The oil I use has very limited selection (Chevron Delo 400). It is available in straight weight, 10-30 and 15-40. Besides not being an "energy conserving" oil, what are the drawbacks to running a thicker oil?
I have heard that thicker oil contributes to the burned valves that Metros seem to have a problem with. Any ideas? Thanks

302:

Some will say that heavy oil doesn't allow the hydraulic lifters to bleed down correctly, which doesn't allow the valves to close all the way, which contributes to the valves burning. I don't know the exact specifics of this, but some people on the Geo forums are pretty sure about this. Check out this website for a discussion about oil:
http://www.boss302.com/oil.htm

5W-30 is preferred, 10W-30 is fine as long as it doesn't go below freezing. They originally said 10W-40 was okay but later determined that it was one contributing factor to the burned valves. If you're getting that oil free, go ahead and use the 10W-30. If not, I see no reason to use it, it's nothing special, any synthetic would be a better oil and a 5W first number will do a better job on startup.