I changed to Amsoil sysnthetic gear oil in my 4 speed and it did not shift correctly. Honestly it's been so long ago and I left it in for a short time only, so I forgot exactly what my symptoms were. I think it was just hard shifting. I called Jamie Passom and he told me the old 833 needed regular gear oil the make the synchros work properly. He told the Amsoil was too slippery.