When I do head gaskets & valve lash I raise the front of the mower about 6" to 1'
Even 4" ( a couple of house bricks ) under the front wheels will stop the oil making a mess
Noe if that oil is thin & smells like fuel then you have a carburettor float valve not working .
By the time you get done you will be ready to start posting you tube videos .
Do not get too prissy about the "tightness" of the feeler gauge
What I usually do is slip it in then tighten the gap so the gauge can not fall out then back off slowly till it slides out under the weight of the whole set .
Way back I was at an auto show where there was a competition stand where people could set a gap with a set of feelers which was then checked electronically
About 5% of the people who tried got it correct to within 0.0001 so went in the draw for a new car and less than 20% got it to within .0005" so got a pack of detailing products .