Of course, this does not mean that modern engine design principles have eliminated oil leaks- far from it, but with a few exceptions, oil leaks through rocker cover gaskets on modern engines are usually traceable to incorrect installation of parts during repair procedures that may not originally have involved oil leaks. Let us look at how this relates to rocker cover gaskets, starting with-
Incorrect or sloppy installation
Although rocker cover gaskets are not service items in the sense that they have to be replaced at regular intervals, some repair procedures, such as repairs or service to camshafts or other valve train components require that rocker covers be removed from the engine.
While this does not necessarily require that the rocker cover gasket be replaced upon reassembly of the engine, it often happens that a well-used rocker cover gasket shrinks when it is not subjected to a clamping force, such as when the rocker cover is bolted to the cylinder head. In practice, however, it is not possible to determine whether (or not) any given rocker cover gasket has indeed retracted during disassembly just by looking at it.
So, here is the problem; many mechanics clean off all traces of oil from the gasket’s sealing surface upon reassembly of the engine. While this is not necessarily a bad thing, the problem is that when the rocker cover is installed and bolted down, the lack of lubrication between the mating surfaces of the gasket and the cylinder head often prevents the rocker cover gasket from expanding uniformly when the retaining bolts are tightened- even if they are tightened correctly.
Thus, while it might appear as if the rocker cover gasket is clamped down evenly, it is subjected to uneven loads because of uneven expansion caused by a lack of lubrication, although this does not always cause the gasket to leak. However, when the rocker cover gasket does leak, no amount of tightening of the retaining bolts will stop the leak. As a practical matter, loosening and retightening bolts in an attempt to repair the leak will just disturb the gasket even more, thereby increasing the severity of the leak.
The best way to prevent this kind of situation when re-using a rocker cover gasket is to apply a thin (!) layer of clean lubricating oil or petroleum jelly to both the rocker cover gasket and the sealing surface of the cylinder head to prevent the gasket from expanding unevenly during tightening. Note that this also applies to new rocker cover gaskets.
The use of sealants
Modern rocker cover gaskets are highly engineered components that are made from advanced materials such as Viton, stabilised silicone, and purpose-designed polymers that are designed to provide a positive seal under high pressures and temperatures.
Moreover, these advanced sealing materials work best when they are in direct contact with both sides of the joint, which means that the presence of sealants like silicone actually prevents the rocker cover gasket from creating a positive seal between the cylinder head and the rocker cover.
The use of some aftermarket gaskets
While many aftermarket seals and gaskets, including rocker cover gaskets, are OEM-equivalent in terms of form, fit and function, it is also true that some aftermarket parts are at best, poor quality knock-offs of OEM, or OEM-equivalent parts.
Unfortunately, many poor-quality parts are so well-made that it is impossible to judge their quality just by looking at them, so it often happens that a consumer only realises that they had bought a knock-off when the "bargain-priced" gasket does not quite fit the contours of the rocker cover. While forcing and/or massaging such a gasket into position in the rocker cover might solve an immediate problem, i.e., making it fit the rocker over, there is no telling how such a gasket will perform when it is clamped down, or when it heats up for the first time.
In practice, such a gasket is grossly misformed, and if one adds the fact that a poorly fitting gasket is almost certainly made from recycled rubber that rapidly shrinks under high temperatures and chemical attack from hydrocarbons, an oil leak through that gasket is all but guaranteed.
Therefore, the best thing to do is to replace rocker cover gaskets only with parts supplied by reputable suppliers, or with OEM or OEM-equivalent parts to not only ensure proper fitment but also to ensure a leak-proof seal between the rocker cover and the engine.
As stated elsewhere, rocker cover gaskets are not service items, and unless a rocker cover is removed or disturbed in some way, most rocker cover gaskets will provide reliable service and operation for many years.
However, up to about 15 years ago, most flexible seals and gaskets, including rocker cover gaskets, contained a significant percentage of natural rubber, which is not immune to the effects of chemical attack by the hydrocarbons in engine oil.
In practice, this meant that over time, the natural rubber in the gasket was gradually dissolved by contact with engine oil. The effect of this was that the gasket material gradually shrank, hardened, and degraded, which eventually destroyed its ability to provide a positive seal between the rocker cover and the engine, thus allowing oil to seep through cracks and splits in the gasket material.