15c temp gap between E8400 cores

So i've had this cpu for a year now and i realize that i got from 10c to sometimes 20c temp gap between core 0 and core 1. I have Zalman 9500 hsf and i use mx-2 thermal grease on both. I managed to lap both the cpu and the hsf pretty well hoping this would fix the problem or so, but no luck. I re-mount and re-applied the thermal grease 3 times but still getting around the same temps:

Idle
core 0: ~20c
core 1: ~35c

load 100%
core 0: ~35c
core 1: ~45c

I use realtemp latest version. I have never overclocked the chip. I do'nt know if it would be a good idea.
I really need some help here.

the temp sensors are only so accurate , they only need to know when to throttle chip down not how balmy it is in ure room, if you were to oc then you should look at higher core temp tho.