If I am reading your post right you are fabricating & welding oil tanks; not working on tanks that have oil in them while welding?

Without trying to be a smart aleck, my first thought is "If it ain't broke, don't fix it". On the surface, it doesn't sound like there is any problem, especially if your customers have been happy with your tanks and if you are making a decent profit.

But there is always room for improvement. I'd say first of all to check the welding code (UL-80?)you are working with to see what allowances, or essential variables, that you need to stay within. Then I would do a little experimenting to see if improvements can be made and what the real gains will be. Salesmen can provide a good bit of insight on what is on the market that might help with improvements (and naturally they want you to spend your money). You may find that a new cutting machine or welding machine will pay for itself by allowing better process control.
But it is also a good possibility that your process and methods are fine as they are.

Chet Guilford