I have used Waxoyl many times in the past and have found the following:
It's ok for about 2 years provided it has been properly applied (i.e. heated up to reduce its viscosity and atomized application with a spray gun). However, there is a real risk that it will not creep into all the joint gaps because it is not particularly thin. This means that it seals around the joints rather than penetrating them so sealing any moisture (or existing corrosion) in and so increasing the rate of corrosion rather than preventing it.
The next problem is that over time (2 + years) Waxoyl gets harder and can crack so letting in moisture and causing increased rates of corrosion. As stated before, it can also block up drainage holes so trapping in water.
In the past I have also used thinned down old engine oil following an oil change but its messy and not particularly environmentally friendly.
I have recently used a sacrificial type of protection on my 911 from the Rust stop shop (they advertise in Porsche Post) which is extremely thin (bit like a slightly thicker WD40) so allowing it to creep more effectively into joints and difficult to get to box sections. It contains a rust inhibitor and like WD40 it chases off moisture. Once applied any water forms globules and simply runs of it (bit like a good wax polish). As it remains fluid it does not crack with time. Because its sacrificial (i.e. it is gradually washed away) it needs reapplying every year or two (when water is seen not to form globules on it). It can be used over the top of Waxoyl as it will penetrate any cracks but obviously if any moisture has already been sealed in it won't penetrate those areas. The only warning I would give is that if applied heavily it drips off on your nice garage floor or driveway.
In truth, nothing really stops corrosion altogether but I think a thin fluid film is preferable to a thick waxy product.
