I'm not so sure that source was correct on the GL-4 80W-90 being the correct lube for the Teralow when installed in a 231, my bet is it was written around a gear based tcase like the Dana 20. Tera's .pdf for install directions for the 231 Teralow has been unavailable all day so I wasn't able to verify it. I think you guys are correct on it requiring ATF when installed into a 231. Sorry for what I now believe was some incorrect info I posted earlier. It was bothering me since I knew the correct lube for a stock 231 is ATF.