In general terms, it's better to flow up than down, simply to purge air from the exchanger. Oil pumps move so much fluid that it might not be an issue in your particular install, but there have been lots of cases where significant portions of heat exchangers were 'air locked' and doing nothing to cool the fluid going through them. In non-Lyc installations, if both inlet and outlet happen to be on the bottom of the HX as mounted, it's common to install a small air bleed from the top of the HX top tank back to the reservoir.
I haven't heard of many, if any, Lyc oil coolers being mounted with horizontal flow & both ports on the bottom, but if it's done, there's a risk of 'air locking' the exchanger, at least to some extent. If 20% of the HX just has air, then you lost 20% of your cooling.
Charlie