It all depends on a few things, Temperature for an example. Colder=Thiner oil. Plus you have to take into account the age of the engine, the older it is the bigger the clearances are in between the rod and main bearings. You can compensate for a little bit of that with thicker oil, just dont go overboard

 

i use FUCH 5 -30 w fully synthetic. but live in UK ( its made by silkolene). The 5 is viscosity when cold so 5-30 or 10-30 doesnt matter as its 30w at full temp anyway, just thinner when cold. I would recomend lucas stabaliser to keep lifters full, but try 5-30 first alone as it should fill lifter better on start being thinner.

How much does pressure fluctuate? mine has done 60000 miles and is at just below half on idle and goes up with revs. when oil gets crappy pressure deminishes so get it changed. let us know how you go on!