Haven't really studied gasoline engines on how EGR be worse or better on oil, but have dealt with EGR equipped diesels since the EPA started that mandate and it is absolutely brutal on engine oil. Of course there is a lot more soot and other issues going on..... but,
If recycling exhaust gasses back into any engine is a good thing, think of it this way... next time you take a dump, then grab a handful and re-eat it and then your next dump should come out cleaner than the first one!! Not exactly the greatest analogy, but should get the point across.
It may reduce Nitrous Oxide, which may or may not be the great climate change culprit, but either way, I find it hard to believe that it is a great thing for the engine and the oil. Does anyone ever wonder why there is no EGR on high performance racing motors or non-road use construction. ag, and marine equipment? Could it be that EGR is not a great thing to do to an engine?
EGR effectively lowers the displacement of a street engine by filling cylinders with some inert (non combustible at least) gas left over from the combustion process. The last thing a racecar wants is to lower its displacement.
More accurately it lowers the combustion temperatures. The recirculated gas is essentially inert.
NOX is formed by combustion temperatures getting high enough that the normally inert Nitrogen portion of the air gets scorched. EGR cuts peak temperatures enough to greatly reduce it.
A large share of the drop in power output can be gained back by proper tuning. With EGR induced, ignition timing can be increased and fuel mixture increased.
It isn't the NOX portion of the recirculated exhaust that can cause oil problems, as there isn't much left at this point.
Actually, there isn't much damage to oil from the EGR other than what slight exchange there may be through the PCV system, especially during low vacuum or intake valve reversion when there is some pulsation in the intake manifold.
The greatest issue is probably after an engine has had enough use that there is a lot of carbon build up in the intake ports near the intake valves. This build up is mostly from reversion, that is a little flame wash back when the valves first open when a bit of flame jumps into the intake manifold. When there is a lot of carbon, more fuel is likely to fall out of suspension and is more likely than not to end up in the crankcase.
EGR allows you to trump the combustion triangle between HC/CO/NOX. It keeps the combustion temps below the NOX production temps (at least where it really takes off).
It actually allows higher performance under emissions compliance since detonation is inhibited by the cooling of combustion temps.
In a gas engine, I'd say it a neutral component from the oil's stand point. If it wasn't used (in one form or another - my jeeps and my 3.0 Mitsubishi used cam timing to leave enough spent stuff in the combustion chambers to do the job) you would see reduced compression ratios and retarded timing probably being the coping mechanism of choice. That's what we saw before it became wildly adopted with (I think) GM being the first to install it.
Edit: I see Big Jim said it all while I took a nature break
My previous car was a 1998 Toyota Camry V6 and it had an EGR value that needed to be replaced. This was the only non-routine maintenance I paid for in 11 years that I owned the car (I count brakes as routine maintenance).
Anyway, this particular engine was know to be brutal on oil, and many cases of sludge appeared. A class action suit was filed and Toyota ended up extending the warranty.
I used M1 5W-30 from the first oil change on that car and obviously had no sludge problems whatsoever. Engine ran perfectly (good as new) when I sold it recently. If you use a full synthetic, I don't think you need to worry about an EGR, but I would wary of extended OCI beyond 7500 with synthetic.
Most cars have passive or active EGR systems. By cam timing, actual valves, or even with help from bumps inside the exhaust manifold.
Since it ultimately keeps the engine combustion temps a bit cooler, it should make the oil have an easier life.