Not sure how to post a link to a prevous discussion on this subject

so I have copied/pasted an earlier reply on this subject, perhaps somebody with forum savey could post the link.
The Z transmission was also over engineered, compare the massive wide gears with those from more modern gears in the later transmissions, the Z gears are twice the width. Many of the oils intended for cars where never designed to be used in a gearbox. The oil you choose should be one that has passed well in gear performance tests, after all the transmission is probably the most expensive part of the engine to reclaim because of a poor choice in your oil. To examine gear oil performance the ASTM test methodology D-5182 (FZG) is used. In this test, two hardened steel spur gears are partially immersed in the oil to be tested, the oil is maintained at a constant 90 degrees a a predetermined load is placed on the pinion gear,and rotated at 1450rpm for 21700 revolutuons , the gears are then tested for scuffing, each oil is rated on how much scuffing has occured, if the gear has more than 20mm of wear along width the gear tooth the test is ended and the oil has failed the test, if there is less than 20mm of wear the test is repeated with additional load. Each time additional load is added, the test oil advances to a higher stage, the highest stage is 13.
6 oils passed stage 13 with no scuffing wear at all. These where Amsoil, Bellray, Maxima Maxim 4, Valvoline 4 stroke, Royal Purple and Lucas HP.
Another consideration for your choice in oil is the high variable of operating temperture, most oils are designed for water cooled engines where the engine temperture is much more stable, aircooled engines will demand an oil that can withstand higher tempertures and have high shear viscosity stability.
A simple test you can carry out on your current choice of oil is to see what condition the oil is when you come to carry out an oil change. A good oil will look similar to when you poored it in, so if its gloopy and brown when it goes in and black and thin when it comes out, change your oil more often or change to another oil. we see some popular car oils which are black and broken down even after just 500 miles. Oil choice will become more important to you as more of the original Z transmission parts become unavailable from Kawasaki. Dont be fooled into thinking that because its an old bike and you dont thrash it then any oil wll do.