hi guys, i found out my car rev and speed go down quickly when i lift off the gas pedal while on gear.. I am not sure, but could this be the spark plugs or oil?? I have seen the oil a bit dark and will have to change it.. what oil can i use?

also the cruise control seems to switch off after a while, mostly when going slighty down an angle... when I'm on a flat road and switch the cruise off, the speed tends to decrease as if i have moved to lower gear....

could these two be linked???

thanks