The Yamaha XJ750J, also known as the Maxim, is a cruiser whereas the XJ750RJ, the SECA, has more of an European racer style. They have the same engine but have a different gas tank, body panels and seat.
the answer is on the inside of the cover plate on the left side (sitting on the bike)cover the triangular cover. I have a 1982 Yamaha xj750j and it says sae 20w/40(above 5 c)sae 10w/30 (below 15c)
Front - 3.25H19-4PR Tubeless Rear - 120/90-18 65H Tubeless
The oil fill is on the right side of the engine below the carb on the upper part of the case.
Its located on the very front of the motor down by the exhaust pipes.It has a round housing and it takes a 12 mm to get it off..
the difference is the Yamaha is better
The only difference I see is the di version includes the ac power adapter.
The timing in the CDI unit.
the numbers
The Yamaha U3 is slightly larger than the Yamaha U1, this is the most significant difference. The larger size of the U3 results in longer strings and consequently to a richer and warmer sound.
about 25 to 35lbs depending on the year.
J is 1982, K is 1983, carbs are the only main difference I think.
The 620 is large bore while the 640 is medium bore.