We investigated the possibility of reaching nanojoule-level pulse energies in a femtosecond erbium-doped fiber Mamyshev oscillator. In experiments, lasers generate stable pulse trains with energy up to 31.3 nJ, which is comparable to the highest achieved by prior ultrafast erbium fiber lasers. The pulse duration after a grating compressor is around 100 fs. However, as the pulse energy increases, the pulse quality degrades significantly, with a substantial fraction of the energy going into a picosecond pedestal. Numerical simulations agree with the experimental observations, and allow us to identify the gain spectrum and the nonlinearity of the erbium-doped fibers as challenges to the operation of such oscillators at high pulse energy.