Having been down for years, this log would not be a good candidate for edible mushroom cultivation. Although it may not be visible, any logs left over several months or more will have already been colonized inside by wild fungi. If you then inoculate with Shiitake, Oyster or another mushroom of your choice, your mushroom will be competing for space (food resources) in the log. You might still have success producing some mushrooms, however for best results start with logs from healthy trees that are freshly cut or fallen. That said, if the log is there and you love to experiment, oyster mushrooms would be the best choice to plant because they grow very quickly and aggressively. Another good candidate would be Turkey Tail, a beautiful medicinal fungi which sometimes be found fruiting in the wild on older logs that have lost their bark.