I used the word "informal" to describe their sparring sessions, but it appears that I should have been more specific. It's important to remember that Bruce Lee would spar with anyone and nature of the sparring sessions were quite eclectic. They basically fall into 3 categories.
PLAY SPARRING: This usually applied to people not involved in the martial arts or those were not well known in the martial arts world. For example, Bruce once sparred with Ryan O'Neal near his trailer on a movie company lot. Ryan was a former boxer and did not believe in the effectiveness of the martial arts. Bruce coaxed O'Neil into moving around and throwing light punches at one another. This form of play sparring quickly got out of hand when Ryan began to get too close to Lee's chin with a few punches, so Bruce side kicked him into the side of his trailer. Bruce apologized to Ryan, and despite his history of having a bad temper, O'Neal was cool about it.
POINT SPARRING: This appears to be the kind of sparring that Bruce engaged in with Tom. Bruce sparred in this fashion with Chuck Norris, Louis Delgado, Bob Wall, Oakland student Leo Fong, a few of George Dillman's top students, Chinatown students Steve Golden/Larry Hartsell, and many others.
FULL CONTACT: Every Sunday at his Chinatown kwoon, Bruce sparred with Dan Inosanto and/or Dan Lee. At his home in Culver City, Bruce would do the same with student Ted Wong. These sparring sessions would involve wearing full body gear, the same gear Bruce used at the 1967 Long Beach Internationals. Despite never sparring one another, Joe Lewis admitted that "Bruce liked contact," and Dan Inosanto told a surprised Lewis that Bruce "was tough."