Having issues with OnChange Scripts

I've been trying solutions recommended by similar posts on the forums, verifying to the best of my knowledge that my Javascript syntax is correct, and I'm still having issues.

I'm trying to display a field "Duration of Rental" only if the Opportunity Type "Rental Quote" is selected.

Currently, I have, in the Opportunity Type OnChange Script:

if(oppo_opptype.value != 'RQ')
{
oppo_rental1.style.visibility = "hidden";
}

RQ is the lookup code for "Rental Quote" in the lookup table this field uses. I've also tried "this.value" and a few other combinations of if/else statements and I can't get a single response when I make changes to my test record. Please help