Whether you need shims or not depends entirely on the mesh between the teeth of the magneto drive gear and the intermediate gear when the mag straps are fully tightened...
This can vary due to production tolerances and the degree of wear on the crankcase (magneto platform) where the magneto sits....
Sometimes this is unworn or has only light wear...If when the mag straps are tightened the gears are meshed too tightly brass shims of .005" thickness(made for the job and available from some magneto overhaulers) are fitted under the mag as required until the correct mesh is achieved...There should be a small but discernible amount of 'backlash' between the gear teeth....
Hold the intermediate gear by hand to stop it moving and test the backlash by trying the mag drive gear for radial movement at the point of mesh...
In extreme cases the mag platform may be badly worn and the problem is then more difficult to rectify...Bad wear occurs when the bike has been run for extended periods with the mag straps not fully tightened...
The original fibre/rubber seal that fits in the casing behind the mag drive gear and seals on the shoulder on the back of the gear can be replaced with a modern 'sprung' oil seal which is far more effective...
However, it is also possible for the hole into which the seal fits to be worn as well...Badly worn seal housings cannot be accurately rectified without some machining work when the engine is stripped...The only solution (bodge) that may solve the problem to a degree is to stick the seal in place...There are definite limits to the accuracy and success of this method though...
The condition of the shoulder (on the mag drive gear) can also be a factor...A worn or corroded surface will also compromise the seal...New mag drive gears are available though...
As oil loss was the original problem some or all of the things noted above will probably be the cause...Ian 