95 formula, 112k miles, 6spd, all stock except for muffler, trickflow elbow and kn air filter


just recently switched from 10weight to 5weight on the engine oil and it feels a bit sluggish now.... and im noticing my mpg went down from an avg of 15.5 down to 13.1

does going to lower weighted oil really effect my engine like that?

I live in deep south texas....sorching sun and wind....hot all around, im sure the heat may have something to do with it

Should i go back to a heavier weighted oil?

that chihuahua looks like he's got everything under control, but ditto to that about the weight oil... i live in el paso tx, summer's are brutal with hardly any moisture in the air and i too used 10w-30 Mobil 1 full synthetic at that. with 5w-30 u might have to be topping it off every now and then depending on how you drive too...yeah id make the switch back even though it says 5w-30 on the oil cap...i think that compensates for those owners who live in Iowa or Minnesota

Not jus weight, but oil brand affects performance. Oil must 1) cool, 2) seal, 3) lubricate, 4) clean and 5) prevent corrosion. However, there is a 6th attribute, it also must stay where it is put and this is most crucial with a hydraulically operated valve. If the oil thins out when hot, doesn't maintain viscosity, breaks down early and runs off of surfaces and is built of short chain molecules (like synthetics)...in short, it doesn't last very long--the engine will run badly.

In my experience, the max a good oil will last is around 3,000 miles (Castrol GTX, Union 76, Standard or Chevron). Other oils like Quacker State and Penzoil break down within about 600-750 miles. With bad oils, break down will be felt with a rough running engine caused by the lifters deflating.

5W-30 oil is built on a 5 weight stock and then uses plastics that glob together to increase viscosity with heat. 10W-30 uses a 10 weight base and thus less plastics to increase viscosity. 10W-40 uses way too many plastics and is not recommended for the LT1 engine. 20W-50 is not recommended either--apparently, becuz even though it starts with a 20 weight, getting it to 50 weight requires lots of plastics. "W" means rated for winter and not weight or viscosity.

There is lots of info available about oils and what is in them on the Internet. Some of the most popular oils are popular only becuz they advertise a lot (like Penz and Quacker State). When they are analysed, they don't do very well.