i personally think its ok to manually shift through the gears as long as you have common sense and don't put it into d3 when you're going 90 mph. but although it will be ok, i hardly do it cus its not worth the extra effort and theres no need to act like you have a 5-spd. if you want a 5-spd that bad, do a tranny swap...
btw the only thing i do now is just switch from d3 to d4.
keep it in d3 for city driving and d4 for speeds over 45 mph.
as long as you're smart about it and know how the engine and tranny work you will be fine. :thumbup: