Hi, had my 528i for a few weeks now and was going to top up oil,but,dont know what type is in there atm so i will drain and refill. Firstly can i replace the oil without changing filter? and secondly i plan to use Castrol Megatronic 10/40,will this be o'k' or should i go for a 20/50? as the car has 210k km on the clock but runs fine, i also plan to use it on my e36 320i aswell,