I RECENTLY CAME ACROSS a couple of blog posts dealing with the question of using the same oven for dairy and meat, or using an oven previously used for non-kosher items. Both noted the somewhat lenient view of R. Moshe Feinstein which differentiates between dry and wet items. His position is that with dry items one need not be concerned about any vapor remaining in the oven that might be capable of transferring flavor to another item, because such vapor is minute and insignificant, while with wet items this is a genuine Kashrus concern that needs to be dealt with accordingly. From personal experience, most people I have spoken to think this view is either normative or a bit too lenient. It seems that most people I have met think that using the same oven for wet meat and milk products, even one after another, is a major leniency not worth relying on.
At the moment I am not prepared to deal with the underlying textual arguments for the opinions I will mention here, but I do wish to mention them nonetheless, so that people be informed.
In Lakewood it is well-known that R. Aharon Kotler held that there is no concern for vapor in modern ovens whatsoever. Similarly, R. Yitzchak Abadi told me that unless one pot is directly above the other and both are uncovered (in which case there would be good reason to concerned that something might spill) one may use the same oven for meat and dairy at the same time, regardless of whether the items in question are dry or wet. Their underlying reasoning is twofold: 1) Tests demonstrate that vapor disappears the moment it enters the airspace of the oven. 2) Any significant vapor that might possibly remain would go straight out the vent anyway. The same reasoning would allow one to use an oven previously used for non-kosher food.
Even those uncomfortable saying that one may use the same oven for meat and milk at the same time are still willing to permit when it is one after another, at least in cases of need, and certainly after the fact. The following appears in a sefer called Meor HaChaim published a few years ago in Lakewood (see אות ב):
As you can see, there are very ‘mainstream’ poskim today who essentially hold that there is no problem whatsoever; even if they rule that one should be machmir lechatchila in many cases. Interestingly enough, the author of this sefer couldn’t stick in that part without a fight. Take a look at R. Yisroel Belsky’s approbation:
Well, he printed it anyway.