The oil fill tube o-ring as used on all EJ engines is an NBR 22.1mm x 3.5mm JIS P22.4 spec. The durometer spec is either 70 or 90. Pricing on this o-ring is around $4, but could be purchased in bulk from specialty suppliers for under $0.15 each.
Subaru OEM part number: 806922120
Bulk options: