"My question concerns the use of food-grade lubricants in enclosed helical and helical-bevel gearing that is primarily splash lubricated. Normally, we specify ISO VG 220 EP lubricants for this type of gearing. However, I understand that food-grade oils do not typically contain the sulfur-phosphorous-based EP packages. What advantages do food-grade PAO/SHC or food-grade PAG lubricants offer over straight mineral or white oils? Are there any advantages in terms of oxidation resistance, anti-wear or scuffing resistance or water demulsibility?"
By food grade, I assume that you mean H1 – an oil that may be prone to incidental contact with food. Under these circumstances, the requirement is an oil that contains only additives appearing on the Food and Drug Administration’s "approved list" for food-safe compounds. Generally, H1 lubricants are formulated with polyalphaolefin (PAO), polyalkylene glycol (PAG) or white oils as the base oil. You are correct that the common sulfur-phosphorus chemistry used in many industrial gear oils does not meet H1 requirements.
H2 lubricants are used on equipment and machine parts in locations where there is no possibility that the lubricant or lubricated surface contacts food. These compounds may be used as a lubricant, release agent or anti-rust film on equipment and machine parts or in closed systems in locations where there is no possibility of the lubricant or lubricated part contacting edible products.
H3 lubricants, also known as soluble or edible oils, are used to clean and prevent rust on hooks, trolleys and similar equipment.
As a gearbox supplier, what is required of course is the ability to adequately lubricate under high-load (boundary) conditions. Provided the H1 lubricant selected meets the required performance properties (OK load, 4-ball wear and weld, FZG, etc.), it should be able to perform acceptably.
In terms of oxidation resistance, synthetic oils (PAO, PAG, etc.) may show slightly better oxidation resistance than mineral oils, while the highly refined nature of white oils means that their long-term oxidative stability should approach that of a synthetic.
Likewise, the demulsibility of white oils and PAO synthetics will generally be better than mineral oils (not withstanding any major differences in the additive package, which can also affect demulsibility) because of their inherent "purity" and absence of polar compounds.
The real issue between food grade and non-food grade is cost, and to a lesser extent performance. Generally speaking, non-food-grade lubricants will provide the same or often better performance at a lower cost. So unless a food-grade lubricant is necessary for the intended application, you are typically better served with a "conventional" non-food-grade product.