Whats the Difference with all these motor oil numbers and colors?

[IZ1111]

I want to get the best thing for my engine, but dont really know whats the deal with all the numbers, and why some are colored in a box and others are just plain. What does it mean?

I used the 10W 30 (with the green box) on my car regularly. But I decided to switch to 10W 40 for a better performance, but I was wrong. its even worse. Now I'm thinking of trying 5W 30, to see if I just went the wrong way with the numbers. But I really would like to know whats the big deal about them.

[denisond3]

The difference between using 10w30 and 10w40 should not cause any change in how the engine runs. The best oil to use might be whatever is printed on the top of the oil filler cap on the top of the valve cover. Here is a URL that will tell a lot more about motor oil than you ever imagined there was to know:
http://www.bobistheoilguy.com/

I think any oil you buy (other than used drain oil) would be fine for your car - but the later model cars often call for 5w30. If you live in Houston, I dont think it ever gets cold enough that 10W30 wouldnt be equally good. An important point is to change it often enough. You can read about that at 'obistheoilguy'.

[IZ1111]

Ok, thanks. But why would 5w 30 be better for older cars. And how did I get the idea that 10w 40 or the higher the number would be better for older cars. Well I guess I have a lot to read. Thanks again.

[denisond3]

I didnt mean to imply that 5w30 oils would be better for older engines - if they werent designed for it. But for cars from the 90s, many of them do call for 5w30. In most cases the owners manuals or service manuals will also say that using 10w30 is fine - if the climate temperatures are not too cold. For instance, 10w30 is okay for my 92 Escort at 35 degrees and warmer. It would NOT be good in Minnesota in January though!
Basically an engine should be able to use the oil it was designed for (and which the maker recommends) from its very first day to it very last day. If it needs thicker oil, then it also needs something in the motor repaired/replaced or rebuilt.
The reason lots of folks will begin using thicker oils in an older or high mileage motor is due to high oil consumption or low oil pressure. I have done it myself. The thicker oil is not a long term solution, but its a lot easier than putting in new piston rings, new bearings, or replacing a worn oil pump.