I am doing 2 implants 30 & 31. #30 has plenty of width for a 5.0 mm diameter. #31 is tight, can do a 5.0 with slight GBR. A 4.5 would fit without need for GBR. I have an Oral Surgeon friend that says he never does less that a 5.0 on a molar. Literature seems to be consistent that a 4.5 is comparable to a 5.0.
Thoughts
My go to fixture for molars is a Straumann TL 4.8 WN though I have placed a lot of Straumann TL 4.1 RN with no problems. I am more concerned about the emergence profile and the WN is only available on the 4.8 but Blue Sky Bio make a 4.1 WN. Because of the sloppy digital components with Blue Sky I have stopped using them and gone to placing the 4.1 RN in such situations. If you read the product literature from Straumann this is why they went to the Roxolid material over their traditional titanium because it is stronger and thus a smaller diameter can be utilized.