Interesting question.
The Lycoming service bulletin is headed "Mandatory". However the S/B detail commences with "Lycoming recommends".
S/B can be found here
http://www.lycoming.textron.com/supp...dfs/SB480E.pdf
I believe that the S/B is not mandatory so you are not breaking any rules if you fly beyond the 50 hours or 4 months.
It might also come down to how the entry is worded in your maintenance release ie. Oil change and filter required at xxxx as distinct from IAW Lycoming S/B Oil change and filter required at xxxx. Just my opinion.
I am keen to see what the engineers think about this.