I cant remember where i read that synthetic causes clutch slippage, but i did see it. I bought a 2003 that the guy ran synthetic in it and the clutch was smoked. as for 10-30 thats what my service manual says to use and my 93 is still going strong. I replaced the diff cover on my truck a month ago and had to squeeze the 80-90 out of the bottle at 10 degree temp. Im not saying im right or youre wrong, just trying to help