............. Just wondering if there's a oil cooler for a non-pressurized oil system?
Yes, aftermarket oil cooler adaptors are available - it has been discussed elsewhere at this forum some time ago. The adaptors mount to the big oil drain plugs on our scooters.
I think using those adaptors might not be a good idea. It looks like the adaptors get mounted at the suction side of the oil pumps. This means the oil needs to be sucked from the oil sump, through both cooler hoses, and through the oil cooler before it reaches the suction side of the oil pump.
I do not trust a systym like that, I think oil coolers should always be mounted on the outlet side of oil pumps - like on all vehicles, trucks, tractors, motorcycles, etc. out there.
I think specially at higher RPM’s the oil pump on a scooter with a cooler system on the suction side of the oil pump might battle to get enough oil to feed oil to other components properly.
Having said that - when I did a Google search to see what the oil cooler adaptors look like, I noticed that at least one seller advised prospective buyers to install a “high flow” oil
pump at the same time when the buyers want to install the oil cooler adaptors.
I think there is a bery good reason behind the sellers advice - an ordinary oil pump will most probably battle to suck sufficiant oil at high engine RPM’s.
The aftermarket high flow oil pumps have wider gears - so I think it should ensure better oil suction through the adaptor, hoses, and oil cooler before the oil get sucked into the oil pump.