I have 2004 SC 430...my local shop recommend me to put in Mobil 1 10w40 synthetic instead of the 5w40 since 10w was little thicker and it was better for the car..
I was wondering if 10w40 was okay on sc430..?? instead of the recommendation of 5w40....will this effect my engine poorly at all..?? or do i need to change the oil
again..?? any advise will be greatly appreciated!!!

Thanks...

I am no expert but I wouldn't think it would hurt anything. I try to follow whatever the manual says. I also just put in the mobil full synthetic, good oil. -Dave

It will probably make no difference but the manufacturer has a reason for recommending a paticular weight. With newer autos the bearing clearences are much closer than in the past thus a "thinner" oil is recommended to improve penetration. I would suggest that you stick with the man. recommendation unless you are operating in very hot temperatures. Hope this helps

NO, it is not ok to use a higher weight oil. ONLY use what is recommended for your size engine. the mechanic wants your car back in his shop ASAP. Heavier oil can disrupt engine balancing and cause low idling issues to occur. Heavy oil and low octane fuels decrease the amount of amps your altenator produces.needless to say, one problems causes another.

Everyone has opinons but not everyone has facts.

Using 10 40 will not effect the car , if anything it will improve the cylinder compression on start up as the car has aged and is not the same specs for tolerance it once was.

a 10 weight oil is only that at cold start until it gets heated to be a 40 weight so how this could cause an issue is beyond me.

I use motul 10 40 and it is the best oil i have ever used hands down.

Low idle issues or alternator output ????? That really is a joke.
If your car is not idling properly you have a bigger problem than oil.
Alternator amperage output is directly associated to rpm so unless the car is not idling proerply it has no bearing on oil weights.
A one step up on cold being slightly thicker is NEVER going to cause a problem . It stays at that viscosity for a few moments before it heats up and gets thicker by virtue of it being oil.

Already using a sythnetic base oil is going to improve the start up efficency more than using a conventional base stock oil . So you are already ahead of the game.