Changed the AT Fluid

I changed the filter and fluid saturday. the pan had a considerable amount of gunck on the magnets and some debris on the filter screen, the fluid was dark brown and smelt pretty bad. didn't taste it so there was no comparing there. But anyway, i put 5 quarts back in, and there is a noticeable difference in the shift and no more shuddering under extreme acceleration. if i put in 5 quarts of new fluid, how much old fluid was still in the stock system?

MICK - coonhunters do it all night long !
1997 Yukon 4x4
1988 XJ Cherokee 4x4
1987 YJ Wrangler 4x4
Future 1994+ Dodge 3500 4x4 Cummins Diesel Flatbed