I use Gain powder and have no problem with clumping. I don't like to use liquid detergents on a regular basis as I believe it promotes buildup. Just put the powdered detergent in the drum with the clothes and it won't clump.
I really like the Sears powdered detergent. It cleans well and is VERY inexpensive compared to the others. regus_patoff posted a link below showing that the Oxi-Clean formula is on sale for half price through October 4. I'm posting it for you in case you're interested.
I have the top loader LG 5101/HW and find that powder detergent do get the clothes cleaner. I was using the Kirkland powder from Costco but haven't been able to find in in the store recently. I was surprised because one of the review sites, maybe Consumer Reports rated it higher than Tide. Anyway, my machine has a little pull out drawer with an insert for the liquid detergent and you take that out when you use powder.
There wasn't any mold or residue from the liquid detergent. I pulled out the drawer and there is really no way to change it for powder detergent. It has a section for detergent wash (doesn't say what kind), softener, and bleach.