This is my second reply to your post. I changed the oil in my z2100 Ferris with a 26 h.p. Briggs with only 5.7 hrs. The oil level was on the full mark on the dip stick before I drained it, however only 1 and 1/2 qt. drained out, keeping in mind this engines oil capacity is 2 qts. When I put the new 2qts. in the engine the dip stick shows that the engine is 1/2 qt. over filled. Is it possible that Briggs put the wrong dip stick in this engine? Will check with B/S by phone tomorrow and post back. Knowing this I think it is possible that your engine was Low on oil......
You can easily end up with several ounces of oil between the filter, oil cooler, bearing surfaces, valve covers, etc. What briggs puts in their manuals is dry volume. and may not correspond with what actually gets replaced.