Iroko is a naturally durable hardwood. You do not need to apply any preservative treatment. Left untreated it will weather to silvery grey over a 12-18 month period.
If you want to delay this weathering process you can apply garden furniture oil to new furniture. This will then need to be reapplied at least annually to maintain the decorative effect. Oiling is a decorative process. The lifespan of the furniture will not materially change by oiling.
All furniture will benefit from an annual clean with warm soapy water and a nylon bristle brush. Do not use a pressure washer as it will damage the timber surface.
You can leave your furniture outdoors all year round. Extended storage in a dry area is not recommended. Never store outdoor wooden furniture in a conservatory.
The timber used has been seasoned for outdoor use and it is important that it does not dry excessively. This promotes cracks and splits in the wood. Left outside rain and moisture in the air will take care of this naturally.
The easiest option is to let the timber weather naturally and leave it where you use it so that you can enjoy it when you want.