For my service, now very immenent, i am going to use synthetic oil. Mobil 1 probably. But my question is what weight oil do you use? I have an EF I6 auto with almost 130 000ks on the clock and seems to be in good working order.

Mobil has: 0W - 40
10W - 30
5W - 50
15W - 50

I know the first number is the weight of the oil, but what is the second number? But, more importantly, like i siad, which would i use.

15/50

i think the second number has something to do with the thiskness (viscosity) of the oil when its at operating temp. the hight the number the thicker the oils is..
im using Penrite HPR30 atm and it seems great

15/50 is the go, if your motor was newer, maybe a lower grade, the second number is viscosity at op temp, the first being cold, and the W standing for weight, not winter as some think.

The second number is a description of the oil's properties when hot...

Think of it like this... a 10W30 oil is a 10-weight oil that will not thin more than a 30-weight oil when it gets hot.

copied and pasted direct from Castrols web site.

A monograde is an oil whose viscosity is defined at only one temperature, either high or low. A multigrade must meet both high and low temperature viscosity requirements simultaneously. This makes multigrades an easy and popular year-round choice for drivers who experience hot summers and harsh winters. They are easily recognized by the dual viscosity designation (i.e. 10W-30 where the 10W is the low temperature, or winter designation and the 30 is the high temperature designation). It is the viscosity modifier additive that produces a thickening effect at high temperatures but is dormant at low temperatures.