What does the owner's manual recommend from new? How is the oil pressure compared to when newer?
I would stay with the recommended weight unless you have a real problem with actual low oil pressure, checked with mechanical gauge.
I might offer an example, I have 189k miles on my 91S (admittedly a different engine), and the V6 oil pressure is still the same (25/30 psi idle hot and 50/55 psi at 3000rpm cruise) as when I bought it used with 40k miles on it, still using the usual 15W/20W-50 oil, with the recommended higher levels of zinc/phosphorus, as used from the date of the purchase, 1994.
The point is, while I admit the weight of these oils most of us use in the 164 are a little higher than what the manual recommends (because of being the very few oils available these days with the higher levels of desired flat tappet antiwear additives) the pressure has not changed/dropped with the high mileage.
So, unless indicated by measured evidence, I wouldn't expect that you would need to change from the owner's manual values.
If the engine has oil leaks from worn seals, you might try the "High Mileage" version of the recommended oil, as those oils are supposed to slightly swell the rubber seals a little to sort of renew their fit, at least potentially slowing leaks.
Disclaimer: of course, others may have a different opinion, based on their own experience.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6
Last edited by Del; 01-12-2018 at 04:52 PM.