No vacuum pulling oil in.
The exhaust valve is laid over so far on a hemi, the oil doesn't pool up around the guide.
Even if it did pull a little oil in, the guides on a hemi could probably use all the lube they can get, lol.

I've seen so many hi-perf engines over the years that didn't have seals on the exhaust side and had no issues w/ oil getting past them. I decided to try it myself and have seen no problems.