I know about lubricant formulation so can give you the short and long answers. The short answer is that provided 10W-30 is compatible with the Westerbeke viscosity requirements, Rotella T5 will be fine.
Performance of top diesel oils are similar, in part because they have the same major claims (API CJ-4 or CK-4) since that's the market. Most performance attributes (deposit, oxidation, corrosion, and wear) are provided by the performance additive package, also called the detergent inhibitor (DI) package. The DI package contains the dispersants, detergents, antiwear, antioxidation, and anti-foam additives. People often talk about 'detergency' but this is a hold over from the distant past when metallic detergent provided the main source of piston cleanliness control in engines. That hasn't been the case for decades. A modern DI can contain 10-15 individual components, and costs millions of dollars to qualify for something like API CK-4. The DI is combined with base oil and viscosity modifier (a polymer that reduces the viscosity change with temperature) to blend a finished oil of the desired viscosity grade. There are four main additive suppliers in the engine oil world: Lubrizol, Chevron Oronite, Infineum, and Afton,. These companies spend hundreds of millions of dollars per year on R&D in this area. This is one reason you don't want to add extra additives, you've just upset a complex chemical system that someone spent millions developing and qualifying.
Base oils: While it used to be true that 'mineral' oils were distilled from crude (solvent refining process) and 'synthetc' where chemically manufactured, that distinction is no longer accurate. Over the past 20 years the use of hydrocracking and isomerization to create lubricating oil has largely replaced the solvent refined base oils which are known as Group I base oils. For the current performance levels, use of these types of base oils, referred to as Group II base oils, is required for cost-performance reasons. In addition, hydroprocessing and isomerization allowed the creation of high viscosity index (VI) base oils for moderate cost (called Group III base oil). VI is a measure of the change in viscosity with temperature. So while it used to be true that synthetics used high cost polyalphaolefins (PAO), these days Group III base oils are used in synthetics with a only few exceptions (extreme low temperature products etc). Thus ‘Synthetic’ is really a marketing term since there isn’t a clear chemical distinction anymore. Industry practice is to use ‘synthetic’ or part synthetic to refer to the incorporation of higher VI base oils (120+VI). These are often required anyway to meet viscometric targets, but a marketer may or may not choose to identify a product as part synthetic depending on where the product falls in their product line.
As I recall, Rotella T5 is a mix of Group II and Group III. Shell has an internal source of Group III made from natural gas (Gas to Liquid-GTL) using Fisher Tropsch processes (very good base oil). DELO 400 uses Group II and Group III as well. Chevron is a leader in the process technology for hydrocracking and isomerization having started use of these types of base oils back in the 1980's. They also license their base oil technology and catalyst to other companies. Chevron brands their base oil as IsoSyn® (a play on isomerization and synthetic). Both oils will have excellent and comparable engine and seal performance. By the way, seal performance is part of the performance requirements for API CK-4 as well as various OEM requirements.
It is likely that your engine has run on a variety of oils meeting a succession of performance categories over the past 30 years. The current generation of oils are backward compatible for 4 cycle diesels and have performance far in excess of what was available in 1987. There should be no concern on your part about seals or other issues.
.) So I’m not interested in boutique oil for the sake of advertising hype. Also, T5 is a synthetic blend, so maybe not as much detergent as full synth?
