I know this is a stupid question but let me explain. I recently got my '88 IROC-Z (LB9). The cap on the valve cover said 5W30. I did an oil change with that oil. After i was done i was checking the dipstick to get the right amount and noticed that stamped on the dipstick said 10W30. So now i'm thinking why in the hell don't my specs match. Secondly which one is the right oil? I don't think theres much of a difference between to 2 grades. However with a motor with 92K on it, it might need the extra thickness of the 10W30. What you guys think? Thanks for any help.

 

10W30 is the most common grade and has been used for many years. In the last couple of decades or so, car makers have started recommending 5W30. The only real difference is that the lower number is the oil's viscosity in cold weather. The lower the number, the easier it will flow when the temperature is cold.

Both grades will work equally well. If you live and drive the car in a northern climate where it gets really cold in the winter, use 5W30.

 

If there is any question pull your owner's manual out. Usually near the back it has the temperature range that each recommended weight of oil is rated for.

My concern for you is that one or the other is not the original for your car. If the cap is wrong then it is not a big deal. It is just a cap. If the dipstick is wrong then how can you guarantee that the oil level is correct? Depending on what brand and style of oil filter you are using you should hold a total of between 4.5 and 5QT of oil. But there is the possibility of a bigger oil pan etc etc.

 

See thats why i'm kind of concerned now. One of them has been switched but which one? Right now i'm running 5W30. It's not freezing cold here but i figure that it'll be ok. If anything the oil will get to where it needs to go a lil faster. But then again it might not seal or lubricate good enough? What do you think?

 

I would use 10w30. Use whatever gives you good oil pressure I use 10w40 but my bearing clearances are slightly bigger than stock.

 

I use 25W50 racing oil but my engine never gets fired up in cold weather either and it stays in a heated garage all winter.

Once your engine is at operating temperature, the higher number is what matters. The lower number is for when the oil is cold. Mobile 1 0W30 synthetic will almost flow like water in freezing temperatures. My 25W50 will flow like molasses in freezing temperatures.

Even though different grades shouldn't be mixed, there's no harm in topping up one grade with another. The viscosity numbers will change but the oils will still mix.

 

Well i just did an oil change so i replaced all the oil with 5W30. So far it runs fine. I just wanted to be sure i don't damage my new toy.

 

You shouldn't damage anything running 5w30. As stated before at operating temp both oils will have the same viscosity. The difference between 5w30 and 10w30 at colder temps are slight. I run 5w30 in mine and my wife's Camaros, and 10w30 in my Corvette and Nova.