Just to be clear on what an Uncle is: Uncle (from Latin: avunculus "little grandfather", the diminutive of avus "grandfather") is a family relationship or kinship, between a person and his or her parent's brother or parent's brother-in-law. A biological uncle is a second degree relative and shares 25% genetic overlap. A great-uncle (sometimes written as great uncle, grand-uncle or grand uncle) is the brother or brother-in-law of one's grandparent. <Wikipedia> Now that we are clear: The definition is provided to give you an idea of how important it is to interview your Uncle(s). Whether he be your parent's brother, or the brother of your grandparent. He/She may have a wealth of information to share with you, so do your project a favor and locate all of your Uncles and set each one of them up for an interview.
If you have multiple Uncles in one family such as your father's 4 brothers, interview them separately, then together as a group. Sometimes an Uncle can help another Uncle to jog his memory on certain events.