I have a built in deep fryer outside. Is it safe to leave oil in it
Assuming that your fryer is protected from the elements, the safety issues for an outdoor fryer are the same as for an indoor one. Fryer oil can be used over and over again. It absolutely does not need to be to be changed every use. Restaurants that use fryers a lot will filter the oil on a daily or near daily basis, but even oil that gets used a lot doesn't get changed daily. Changing the oil is not a matter of safety. While there is some minor long-term health risk associated with eating oil that is past its peak quality, it's not an immediate food safety issue. Since you use your fryer only about once a week, it should be fine for three months perhaps filtering it two or three times. Even the filtering is for quality purposes only, pathogens cannot survive in fryer oil.

The bottom line is that oil that still tastes good is fine to use.