Does CPU temp increase over time?
 

Summary
Operating System
Microsoft Windows 7 Ultimate 64-bit SP1
CPU
AMD Athlon II X4 630: 38 °C
Propus 45nm Technology
RAM
4.00 GB Dual-Channel DDR3 @ 680MHz (7-7-7-20)
Motherboard
ASUSTeK Computer INC. M4A785TD-V EVO (AM3): 40 °C
Graphics
AL1916W (1440x900@60Hz)
AMD Radeon HD 7700 Series
Hard Drives
596GB Western Digital WDC WD6401AALS-00L3B2 ATA Device (SATA): 32 °C
233GB Western Digital WDC WD2500JS-41MVB1 ATA Device (SATA): 46 °C
233GB SAMSUNG SP2504C ATA Device (SATA): 28 °C
Optical Drives
HL-DT-ST DVDRAM GH22NS50 ATA Device
Audio
AMD High Definition Audio Device

CPU is OC to 3.5GHz from 2.8GHz with aftermarket cooler. Vcore is 1.4. I distinctly remember the temps were about mid to late 20's in celsius at idle when I first put it together 3 years ago. Now it's idling between 35-40c. I cleaned my tower the other day, so I don't think it's dust. Do I need to replace the thermal compound? I haven't replaced any since 2010.

It's worth it to take a look to see if your thermal compound has dried out, which could definitely be an attributing factor, if not the only factor causing your core temps to increase.

dust in heatsink, dried out thermal paste^

Yup that white paste is fine as a temporary solution, but dried up after time, sometimes very quickly

Are all thermal compound created equal?

In terms of heat transfer, yes pretty much. In terms of staying that way? no way

I got a couple tubes of OCZ thermal paste + arctic silver AS5 ... back when I actually spent the time building/overclocking rigs this stuff was more or less considered the unversal standard.

Here is a quick round up:

Thermal Compound Roundup - February 2012 | Hardware Secrets

Is the OP only using the thermal paste that came with the aftermarket cooler?

also keep in mind to work with a pea-sized amount
some pre-applied paste are too thick

My favourite is the thick grey stuff Intel used to include in their samples... As far as stuff you can buy, Noctua's is good