The similarity of topology and use cases between the 5G NR SL (Sidelink) and the IEEE 802.11 (Wi-Fi) technologies may provide an opportunity to extend Sidelink into unlicensed bands originally occupied mostly by Wi-Fi devices. However, for proper coexistence, Sidelink unlicensed band (SL-U) devices must adopt the current channelization in this band as well as channel access procedures. Channelization changes lead to the necessity of modifying the SL physical channels and signals to span across typical Wi-Fi 20 MHz channels. The simplest way to align the 5G NR numerology of synchronization sequences is a plain repetition of them in frequency domain to fill the all required bandwidth. However, such approach leads to a sharp increase in the peak-to-average power ratio (PAPR) value. In this paper, we provide a detailed performance analysis of the proposed design of SL-U synchronization sequences that achieve low PAPR values. The characteristics are studied in a non-stationary frequency-selective channel using a real model a power amplifier. It was shown that the use of an unmodified design of SL-U synchronization sequences has a decisive effect on their detection. The presented simulation results demonstrate comparable performance of the modified design of SL-U synchronization sequences with their design in a licensed band.