my 2004 s2000 really seems to have a hard time starting in ~cold weather (between 30 y 40 degrees). I usually have to give it a little gas and it feels a bit rough until it warms up.

is there anything I can do to prevent that? should I switch to 5w30 or 5w20 synthetic oil? I'm using 10w20 (or 30 can't remember) per the recommendation on the manual.

any insight would be appreciated!

 

Experience has shown me that 30 to 40 degrees is just the temp transition point that confuses the heck out of the ECU. A lot also depends upon things like humidity and air pressure at the time, so the actual temp that it does this can vary from place to place. Not all S2000s do this but many do.
These episodes tend to be seasonal and only last for a few mornings starts. The biggest mistake that you can make is to "help it" by giving it gas. It MUST do it on its own. If it stalls out the first try, let it. Start it up again and let it stabilize its own idle speed. DO NOT interfere with the gas till it figures it out. Eventually, it will learn to deal with this situation. "Helping it" only prolongs the issue and it may never resolve. You may simply have a rough warm up till spring comes. Leave it alone and you may be golden in a week.
You should be fine with 10W30 oil in that temp range. Only if you get to -20*C or colder would you consider going with 5W30. It's not the oil that causes this hard start. It's the ECU having a bit of a hard time compensating for the change in outside atmospheric conditions.

 

i haven't started my car in >40 temps in weeks..but it still starts fine and i use 10w30.

it may be premature, but have you checked your battery recently?