I don't know if they are 100% interchangeable. I know if I was testing I would try something like a P14 or Ross with a chronograph to figure out velocities and if it would be safe for a Lee Enfield.
Japanese 7.7x56R ,.303", was interchangeable with British ammo and was mostly exact copies of the British patterns. It is fairly scarce and at least 72 years old so it would be advisable to shoot other .303" ammo.
As to the reduced load 6.5 ammo it was all full power but the powder was switched in the 1930s resulting in a lower charge which is why some were labeled as reduced charge.
In addition to the above ammo the Japanese ordered aircraft MGs and Mauser rifles in 7.9x57 cal.
Japan produced a number of machine guns that were direct copies of the British Lewis (Japanese Type 92 machine gun) and Vickers machine guns including the ammunition. These were primarily used in Navy aircraft. The 7.7mm cartridge used by the Japanese versions of the British guns is a direct copy of the .303 British (7.7×56mmR) rimmed cartridge and is distinctly different from the 7.7×58mm Arisaka rimless and 7.7×58mm Type 92 semi-rimmed cartridges used in other Japanese machine guns and rifles.