Replace those cracked or perished grommets.
These sizes are available.
Black PVC 19.6mm Round Cable Grommet for Maximum of 15.5mm Cable Diameter
Black PVC 20mm Round Cable Grommet for Maximum of 15.5mm Cable Diameter
Black PVC 25mm Round Cable Grommet for Maximum of 18.5mm Cable Diameter
PVC standard grommet is oil and grease resistant and those made from thermoplastic rubber are temperature resistant. Suitable for fuel lines, electrical wiring etc.