Hello johngladstone,
In my opinion, and experience from other industry groups, it would be fully feasible to form a group or make a system for bench-marking costs for maintnance in 'oil industry'.
It would however, to have any significant value, to be very detailed and specified. A multitude of cost, equipment, resource, quality/age/type of factory units, labour cost and types etc. would have to be defined and sorted out for each company. Book-keeping would be different or even non-existent for some of the necessary definitions, and will require extra preparation costs.
Others may know if comparisons has already been done in some form, valid for the 'production unit' you would belong to.
Secondly the oil industry differs worldwide, and is currently starting a process of implying 'internet of things'. This will again lead to a different focus for investments, and better maintenance with less costs both for maintenance and operation. This will complicate comparisons, but could also be included as parameters for comparisons.
Maintenance costs tend to be regarded as indirect cost. The reason is that because of lack of control and good maintenance policy, theese costs will often occur irregularily, unplanned and unexpected. With a good system, which you obviously is reaching for, the cost will have the opposite characteristics. Hence, in my opinion, to be ragarded as direct costs, as maintenance mainly is adirect result of production amount.
Anyway, more important than book-keping details, is a good planning system where each single component is logged with all technical data and history.
Hope others can contribute with more direct useable information!