Originally Posted by jabba974
At all costs..avoid pensoil....Not nice what comes out of a pensoil motor at the rebuilder...
^^^ This. I once got a Camaro from a lady that had used Penzoil from day one. I took the oil filler cap off and there was jellied crud filling the heads so much I couldn't see some of the rockers. I could just see the gel wiggling where the valves should be. I flushed it and filled it with Castrol and the gel went away. The next time I flushed it and big chunks of crud fell off the inside of the valve cover and plugged the oil drain holes in the heads. I had to pull the valve covers and clean everything. Quaker State is the same junk they even used to use the same bottles just different colors. (Pennsylvania Oil Company = Penzoil, Pennsylvania is the Quaker State)
I use Rotella 15-40 now. IMS Jeep recommends 10-30 for 100 degrees and down and 10-40 for something low like 20 or 30 degrees and up.
Most of the reason that oil turns black over time is the additive package breaking down or getting used up. It's the loss of additives that makes you change oil faster for stop and go city driving; the short drive cycles leave more condensed blow by in the crankcase. Physical shearing and thermal stress will break down the hydrocarbons in oil but not that much. The reason that oil seems to get thinner over time is that the viscosity modifiers that they use to make a 10 weight base oil thicken up to 30 to get 10W30 are not as resistant to shear and break down.
I've heard that synthetics may cause leaks on older engines but really there aren't that many places to leak oil: valve cover, oil pan, rear main, timing cover seal, oil filter adapter (to a much much lesser extent the distributor and head gasket). You could easily replace all of them in a day if you really wanted to.
If you want to know about oil http://www.bobistheoilguy.com/
is a good resource.