Re: proper 70cc oil mix?
Engine size does not matter much for the Chengines. Your 67cc uses the same ratio as mentioned in all the other oil ratio threads.
Oil brand/type, climate, load, jetting and the engine design itself makes more difference than the engine size in this case.
2.6 oz/gal is 50:1 - this would work with a broken in engine and a very good oil, but not necessarily low cost oil.
What brand is this Lowe's oil?