FFC and Puzo had a difficult time creating a scene between Vito and Michael, to show love and affection, so they had to call in the "script doctor," Robert Towne - hence, the garden scene that garnered much attention.

However, no script doctor was needed to show how much Michael loved his older brother Santino. When Michael was leaving to meet Sollozzo, he looked at and embraced his brother with such warm affection that it was palpitating. Then he did the same with Tom (although we saw the "affection" part between Tom and Michael already at the wedding).

Actually there was a significant difference in the way Michael embraced Sonny and Tom. With Sonny it was a full hug, while
with Tom, Michael put his hands around Tom's neck. I always thought it was Michael delineating between his "real" brother and Tom.

But we saw real love between Tom and Michael at the wedding, when Tom saw Mike and Kay sitting at the table. FFC maybe didn't want to be redundant with those two at the 'goodbye' scene.

However, even though it was a quicker hug to Tom, the same warm look was in Michael's eyes.