Hello,
I have a small leak of oil (around 2 or 3 small drops per day) and I cannot find the location of the leak.

I've replace all the valves covers gaskets or seals but it seems to come from the motor. The oil is visible near the starter and drops down at the bottom of the transmission.

My research on the web reveal some problems with the rear crank seal or the oil pan seal.
How can I be sure?
Does one of you had the same or similar problem?
Thanks,

All of my vehicles had some degree of what your talking about.Mileage 120k up to 200+K mi - ALL of the last 6 improved significantly , and saving $$$$ , by using Hi-mi oil . Specifically I like Valvoline MaxLife they been at it in the beginning and forefront I believe - if not the "Better" product out there. My 2 cents anyway , you may be satisfied with your results - YMMV ?
You may have to patiently wait a month or longer.

The rear seal is most probably your issue since you said you've replaced valve cover gaskets, both of my cars had the same small drips that increased over time but both had trans service done at some point and where the rear main seal got replaced, and drips ceased. Both cars also had valve cover gaskets done, the lower heads are notorious for leaks as well as that rear seal.
Most likely a heavier oil MAY help some with the number of drips, but a bad seal is a bad seal and they are cheap in the grand scheme of things so replacement is obviously the smart choice.