Depends on your determination, & age of engines + hours of course. £25,000 engines should last more than a couple of hundred hours. My trucks do that in a month !
Yes, this illusion that buying items from an OEM dealer is always best is often ill founded. In the case o the Mk3 Mondeo parts from Ford dealers are invariably more expensive than the same parts for the Jaguar X-type, those parts being Ford parts with Ford part numbers on them but in Jaguar packaging !
Although this is an old thread, Volvo have continued to drive the price up on these basic cheap looking paper crankcase filters. This year they are £100-110 depending who's selling them......being involved in the aftermarket automotive industry I see this discussion all the time about using Genuine manufacturer parts over OEM pattern parts. Like anything else, it depends what quality you are buying and where you are getting them from. If your engine isn't under warranty you are in no way obliged to use original VP parts, and we all know this is how the Automotive and engine manufacturers make their money, from over priced parts and snake oil.
I agree you need to consider the cost of your engine, but it's also something they rely on to spook you into thinking buying VP parts is the only solution, which simply isn't the case. The likes of VP continue to raise prices and reduce quality so it's not as if they are supplying a top notch product........for the average recreational boater, you are using your engines far less than any vehicle and yet adhering to the same annual service regime with perhaps 60-80 hours of seasonal use depending on how you use your boat. If you are worried then choose VP for certain critical components, but keep in mind Volvo don't make their own oil and they don't make their own parts, they contract them out to suppliers who need to follow a specification, however who also make the same pattern parts, very often with the same or similar quality.
Personally and each to their own, use good quality OEM parts and stick to the VP service schedule and you should have no issues, while walking away with significant savings.
For my kad32's I did buy a VP service kit last year (impeller/oil filter/ engine fuel filter) which was reasonably good value compared to buying the items separately. Also the UJ bellows for the outdrive was not particularly expensive.
I have tried cheaper impellers and found they are not durable so have returned to VP impellers.
I have also found genuine VP belts to be very long lived and require no adjustment. So over time they are good value.
On the other hand I have used Mann and Baldwin oil and fuel filters over more than 10 years now with no issues and the Mann air filers although not so cheap as they used to be are half the price of the VP item.
I have also used aftermarket crank case filters with no issues. On my kad32 is just a can with some breather holes and a paper element inside.
As for oil I have never used VP branded oil.
Belts and rubbers makes total sense to go with higher quality and good to hear you found them much better.
Filters and Oil I'm with you on those, especially once oil grades are met as per VP specification which so many are now. If anyone has any doubts on the drive oil additives where VP claim theirs is superior, this was an interesting read https://www.archoil.se/wp-content/uploads/2012/10/Test case DCCG Volvo- Penta revised.pdf