What kind of transmission fluid can you use for a 1990 Honda Civic 1.5 l MANUAL transmission?

The dealer says to use only 10W30 motor oil I'm surprised a dealer would say that; even though it's a good answer. Honda MTL, which is their Manual Transmission Lubricant, is sold at Honda dealers and is recommended by the Honda literature. I recommend Mobil 1, same weight as you've got in the engine. Usually 5W30 or 10W30.