engine cocktail or recipe for DISASTER??

[daddiojiggy]

let's get past the question of why before we start.i've got 4 litres(each) of 5w30 and 20w40 and i need 7.5 litres for my engine.now i know some of you have jumped ahead but i'll ask the question anyway.....what would happen if i combined the two THOROUGHLY and used the concoction? as an aside i'd like to take an informal poll on oil changes throughout the membership for those that did or still take their cars to dealers for oil changes.i'd like responses to go as follows:
'98 c230 $66.00 aug'01 texas--- where we tell car,cost,date and location .thanks,in advance