Hi;
I'm also searching for an oil cooler, and I see Mishimoto Oil cooler with termostat, but only with 19 row.
Also I searched in Z1 Motorspost o ConceptZPerformance and they had more options (19-25-34Row's, Stillen Core, Braided hoses with silicon protection....) But are more expensive and you have to add the shipping and toll taxes. https://conceptzperformance.com/czp-engine-oil-cooler-kit-nissan-350z-infiniti-g35-czp-21300-xxx3_p_34809.php
In my case I will go to a 25row because of climate in my land, but a Mishimoto 19row probably it's enough in UK.
An option that i've been a couple days ago it's the Hel oil cooler in Tarmac Sport Z
https://www.tarmacsportz.co.uk/hel-performance-nissan-350z-oil-cooler-kit.html
It's a 19row oil cooler for 330pounds I would like if someone have one them for know his/her opinion, but a think that you have to cut and plug the hoses...
What do you think?