We ran a Legends race car for a few years with a 1200 cc Yamaha engine in it everyone was having trouble with the small engines overheating and some blowing up before the 30 lap feature races we ran were over.We were the only race team that was not struggling with a overheating problem, the reason was we ran manufactured recommended motor oil it was a Chinese brand motor oil,I talked to a Yamaha engine rep and engineer he asked why our engine was holding up I told him I ran the same motor oil that the engine builder recommended when we purchased it but had it shipped from overseas.everyone else was running american brand oils, some synthetic.my point is not to altered anything in any manufactured item other than the way it was built and designed.Your asking for trouble.
While that is sometimes true, there are lots of things to be taken into consideration.
When a manufacturer is looking at what specs to build as OEM, and which suppliers to use they are looking at the balance of product quality, reliability, performance, cost to them, and potential profits. All of those factor into a cost/benefit analysis.
Kind of like when i was a competitive cyclist, the idea was that you look at:
1) Low cost
2) Performance
3) Reliability.
And you have to pick two of three. You'll not find all three together.
When people are willing to invest more money in something, they can get a higher level of performance without sacrificing reliability. It's why so many rifles get aftermarket Timney triggers, and why people send so many guns to gunsmiths to get worked over.
We have more options than the manufacturers because we are not bound by the same constraints since we don't have bean counters breathing down our necks to maximize a profit margin. If it costs us another 50% to only see a 10% gain in performance, many of us make that call. We're only limited by our own personal budgets, and sometimes our budgets allow us to use better equipment than the OEM spec.