Hello ando.
At idle, you are probably somewhere in the 600-1000 rpm range.
A guideline from Moyer and others is to run the engine at your normal cruising speed, then aim for oil pressure of 40 psi. This would be for a fully warmed up engine.
Like others on this forum, I ran the A4 for years without a tachometer. But I don't regret installing one (see Moyer's catalog). Other options include a multimeter or timing light with a built-in rpm function. These don't require any additional boat wiring and you get the added benefit of checking timing with the strobe light.
Hope this helps some.
Jack.
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