You mention in your post that, in your, 12 cylinder, 514 rpm, 3.5 MW diesel engines, used as stand-by, have been using a product Mobilgard 450 NC, which is not available any longer.
Prior to change, let us understand the product in use. A web search of the Mobilgard 450 NC, reveals that it is
• “Specifically formulated oil for use with heavily loaded diesel engines manufactured by EMD.”
• Mobilgard™ 450 NC is a non-zinc and non-chlorine lubricant produced with high-quality basestocks and selected additives.
• These combine to provide the oil with low consumption characteristics, high-temperature oxidation resistance and thermal stability.
• Its high alkalinity provides excellent corrosion protection when using fuels containing up to 1.5 per cent sulphur even in the presence of steel, copper, silver and bronze.
• Further, it is an SAE 40 grade with a 13.0 to 13.5 TBN.
Another factor to be taken into account is that, over the last decade the fuel composition has changed dramatically, to meet new environmental norms. As far commercial fuels are concerned, Euro IV and Euro V regimes are in vogue, wherein the sulphur content has reduced, to below 10 ppm. Unfortunately, this stripping of Sulphur from fuels has its own ramifications (http://www.slideshare.net/hussam57/implications-of-ulsd-or-low-sulphur-regime). Thus, it is probable, that you may also need to dose fuel with lubricity additives. Prior to which you may ascertain the lubricity of the fuel from your supplier or have it tested by ( a reputed Lab like, Herguth) for ASTM D 6079 vide High Frequency Reciprocating Rig (HFRR). The recommended value for the scar, must be below 500 mm. Thereafter, you also need to have the fuel tested by ASTM 6079/HFRR with the fuel lubricity additive, at the desired dosage. Compare the two values and ensure the lubricity additive treated fuel, falls in the sub 450 mm scar range, for minimal wear of fuel pumps, injectors etc..
While you have not mentioned the category of fuel, would like to take a cue from the recommended product in use ( 1.5% = 1500 ppm indicated). It has been observed that when we use high TBN oils with low sulphur fuels, there are complications.
• In the marine industry interchange of fuels, from high to low sulphur fuels, results in pronounced piston crown deposits. This is manifest, when using the same ( high) TBN oils. You may consider using a lower TBN oil, if you are now, using LSD/ULSD or similar.
• From the mechanical point of view, the LSD and ULSD generally have lower viscosities, than the residual fuels, therefore modification desirable, in fuel pumps.
• EPA clearance now demand mitigation of Green House Gases (GHG), hence suitable modifications may have to be carried out, in consultations with the OEM to meet, lower emission norms and practice.
In conclusion, your decision to find alternative should encompass some of the above factors.