bluelexus

94 Lexus LS400 noise on start up.

hello , i own a 94 Lexus UCF10. i dont drive the car much. When it sits for about a month or more , and is started there is a clacking noise from the top of the engine like a loose tappet accompanied by rough running. After about 5 seconds everything is good. i have been running a 15w 40 castrol mineral oil. Someone has suggested to use 10w 30 oil and nulon E20 modern engine treatment . The car has done 308,000 k . Any ideas please as to what the noise is and what to do? , thanks , Murray Mules