My car is again starting to slow down now that its warm out. I have always run 5w30, but i've noticed that my manual says it should have 10w30, especially if its above 60*. Why does the 3800 series need 10w30 vs. 5w30. Everyone I know uses 5w30 and has that on their oil cap. I've even noticed that our oil caps say 10w30. I know 10w30 is better for hot weather, but my manual makes it out that the engine can be damaged if 5w30 is used during warm weather. Is this true, and do you think that the 5w30 is causing my car to slow down in warm weather? I use Mobil 1.

I dont know why the manual says it may harm the motor but I belive that your oil preasure is a bit low with 5-30w then 10-30w on a warm day. Maybe this is gm'* fear and that some parts may not get enough lube?
I run 5-30w in the winter (gets to minus 30 here) and 10-30w in the summer (now till nov.)

5w30 won't actually slow my performance, will it?

No I wouldnt see the oil being much of a factor as its thinner when it comes to performance. But the air intake and drawing in warm engine bay heat would diminish performance. How well insulated is your intake and where are you drawing the air from?

Its the stock intake. I have yet to mod my car. It just feels as if i'm towing a 2,000lb trailor. I should really find a way to conquer this ongoing lack of power in hot weather before I mod the car. Moding a improperly tuned car can lead to problems, can't it?

It is better to a mod a well running car but if the mod makes the repair then its all good

WHen was the last time it got a good tune up including, spark plugs, rad flush, fuel filter, pvc valve, tranny flush, etc?

Also hows your tire pressure? if theyre too low it will feel sluggish?

I've done all those things. The only problem is that I replaced the plugs before I joined here, so I had put in Bosch plugs.

How much power loss are we talking about here? Besides noticeble, are the rpm'* higher to achieve the same speed?
Maybe the motor is making the same power and something (tranny?) is holding you back? That is what you say it feels like?

The trans can cause a power loss? Well, actually that makes sence because if I rev it in park, it revs pretty quick. However, it doesn't seem as though the power is getting to the wheels. It shifts fine but bogs down bigtime in TCC lockup. I've driven many GM vehicles that still accelerate fast even after the TCC lockup ( my grandma'* 87' Bonny for example). The gears shift at the right time too. I don't have a tac so that doesn't help. How hard would it be to instal a tac?

We can check out the tranny without a tach. A tach is easy to install on a dis system you just need the right harness, you dont need one though.

The torque converter stator roller clutch can have two different malfunctions.
The stator assembly freewheels in both directions,the stator assembly remains locked up at all times.

If the engine freely accelerates to high rpm in Neutral, you can assume that the engine and the exhaust system are normal.
From a stand still check for poor performance in Drive and Reverse to help determine if the torque convertor clutch stator is freewheeling at all times.(poor accel from take off).
Performance is normal when accelerating from a standstill but at highway speeds Engine RPM and car speed are limited or restricted, the stator is locked up at all times. Examination of the converter may reveal a blue color from overheating?