My two cents here. I am really surprised you are having trouble only running the 1/8. I would add a 1/2 quart first and see if that makes any difference. I could be that you are pumping the oil up top and it can't drain back fast enough.
I mean you no offense but for the life of me I can't understand why people run high volume oil pumps in a SBC. I can't imagine the clearances you would have to have to run out of volume with a std. pump.
Were you having this problem before you changed oitl? If not then I would look at your filter. That is probably not the problem but if it just started, then that and the oil is all that changed.
If you run a vacuum pump or you have much blow-by that is all trying to come to the top where the relief is, and the oil that is up top is trying to get back down. It gets worse if you run vents in the drain holes over the cam, screens over the drain holes at the ends, etc..... Big fight for space between the two. Improving drain back from the heads is a big plus. Just my observations. Many others are better versed than I.
Curtis