I only use Erling seals but even their quality has become questionable. I found that the best seal available was Angus Hawkins (SP?) but they are no longer available, they looked like a 2 piece seal but were not, the seal composition was of 2 materials and 2 colours. Erling are still produced in Germany but mostly in China, Victor Reinze only from Brazil now, and is not what it was when they came from Germany and have had issues regarding quality with them as well. There are several things I do to try and prevent leaks in this area, but have found with time regardless of seal used or attention to detail I have come to accept some will leak with time, you need to remember you are dealing with parts that are over 30 years old. Even when new parts are used you may have problems, had it happen to me with 2 virtually new Brazilian cast FW's, landed up not using either of them in the end, stuck with what works better for me.
This is what I do do prevent/reduce leaks and prolong seal life, on a new engine.
Before I start any work, all parts are super clean as is the work area.
If you look closely at all later AS21 engine casings they have 2 oil return openings to drain oil away from the back of the seal, so if I am not dealing with a AS21 casing I drill a angled 4mm hole under the main bearing as a return, to get the AS21 effect, no need to have oil build up behind the seal, this is done long before any engine building, even befoe I wash the casing.
Next step is to inspect the FW face and get the contact area of the seal cleaned up (the FW gets put onto a lathe and polished) thrust face gets done too.
The round casing edge also gets sharp edges removed with a blade, this prevents the seal from damage when installing it.
The seal then has it's "spring" carefully removed and carefully opened (unscrewed) I then cut about 5mm from it and screw the spring back and refit into the seal, this will increase the tension to create a better seal, the only tool I use here its the side cutters the spring is removed by hand no sharp objects needed.
Before I fit the seal I would have checked the end play and the correct shims and dowels installed, only then will I fit the seal.
Next step is to fit the seal into the casing, I use a proper VW tool for the job, not a hammer and plank as I see many doing.
I smear a litle oil on the outer edge of the seal or on the casing mating surface, the seal is installed deeper than the case edge until the rear of the seal almost touches the casing about 1.5 to 2mm deeper than flush.
I then fit the O-ring to the FW with a little bearing grease, again the qaulity of the O-rings is a problem get the best you can afford, you should look out for a smooth one that is grey in colour, don't try and clean it in any solvent they loose the grey coating and you will have a black seal in the end, I have no idea what the grey coating is, I am no scientest.
Oil is applied to the seal edge as well as around the mating face of the FW.
FW is then slid carefully into position and bolted to spec with some Locktight on the threads.
Take care and your time when doing this, you don't want any premeture damage.
No sealers, RTV or the like is needed, if there is any sealer present in the area the previous builder had no idea what they were doing.
Hope this will help the DIY dudes out there.
Dont know who else to ask for help 
