the 5w-10w rating will not matter because we live in a tropical climate. you should look at the other viscosity number which is the one during operating temps (the second number). kahit magpalit ka ng 5w-30 to 10w-30 eh manipis pa rin ang operating temp viscosity niyan kasi hindi naman nagbago yung "30".

kung sa tingin ng mechanic mo na mas malaki na ang oil journals in between the bearings coz of its age, then probably try a viscosity that is in the 50's range (heck, I've already seen a 10w-60 oil)