isnt 20w-50 synthetic oil?
i am using havoline formula in my civic 10w-30,i asked the person changing oil if i should get 20w-50 synthetic oil,he said no formula is good for your car
n i noticed mostly jdm's use the synthetic oil
i was also using shell's yellow one before
if u wanna flush then just get it from liqui moly, their shop is in f10 markaz next to shell pump, few shops before shahid autos