it is normal i guess for the oil pressure light flikkering at 400rpm,
the lower rpm on thoses is supposed to be 550, lower then that not
enought pressure. Better have your injectors checked, because with
time injectors leaks and that give too much in the engine so this is
the reason your rpm go down, too rich in fuel and poor in air. Do you
have the ''check engine soon'' light on? if so better read codes it
may answer your questions