Originally posted by JD60538:
Not bring a post back from the dead but, I'm sure thre second # is the ammount of detergents in the oil. for example: 5W - 30 means 5weight - 30 detergent grade

detergents are used in engine oils to protect the level of viscosity and to help prevent thermal breakdown.



5W-30 is NOT detergent level. It means 5 Weight under Winter conditions (cold) and 30 Weight under Hot conditions. Also called multi-viscosity. There is a more technical description of the temperatures and methods used for measuring viscosity, but this is good enough for this discussion. What it means that at startup (cold engine) the oil flows well, but remains thick enough at high (warm/hot engine)temperatures to continue lubricating.

Detergent levels (as well as breakdown properties, film strength, etc) are determined by the API rating on the top of the can. these are 2 letter codes ( SF, SG, CC, CD, etc). The "C" ratings are for deisel engines and the "S" ratings for gasoline. In general, engines with oil filters use a detergent oil (detergent keeps dirt suspended in the oil, to be removed by the filter, non filtered engines (most lawn mower engines and alike) need non detergent, so the dirt settles out of the oil.

Finally, as many said in this thread, higher weight oil not only lubricates less (in our engines), but also carries away less heat. Believe it or not, the bearings do not require a lot of oil for lubrication. The most important function for the oil is cooling. Oil cools the bearings and the underside of the pistons. A thicker oil is not able to reach critical areas and does not carry heat as well. Hot bearings=shorter life. It may reduce bearing life significantly.

The original poster had 25 PSI now because his bearings may be significantly more worn than if he had used the 5W-30 recommended.


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