Again..... the search fonction doesn't work so I cant really pull out my reply on a oil thread in the drift section but here is a resume:
The choice of the weight of the oil depends of at what temperature the oil is used at and its ability to hold its viscosity. in 10w30 the 30 states the viscosity at x temperature and the 10 w is how low it can go and still have a viscosity, think of it as a usuable band. So the only difference between a 5w30 and a 10w30 is the ability to hold his viscosity to lower temps then the 10w.
Example
I don have the real numbers at the moment) 10w30 has the weight of 30x from -30 to 100celsius, so under 30 its heavier, and over 100 its lighter. Now I track my car and I know that my oil is getting the shit beated out of him.
Since I know my oil is going to pass the 100 celsius mark, which demandes a heavier oil to have the same amount of viscosity as if I was driving daily on 10w30 and I know that the 15w protects to around which summer never gets really lower then 14 celsius . 5w would becme overkill for summer, since it would probably protect till -40. The logic choice would be from 5w to 40 to 15w50 depending how much you beat the car.
Think about it. Japanese drifters run really heavy oil and most of the time they last longer then american tuned motors which run 10w30.
oh and a used motor has always more free tolerances. So heavier is better. oh and if you daily it...well yes the heavier oil are overkill, but not dangerous for your engine. they will be less efficient at low temperatures.
And ranger whats wrong with the filter?
p.s. race engine or not, if its tracked go heavier. The important thing is not what is used, but how its used.