Hi Tony
As those S1 engine's aged they hardened the seals around the inlet valves and then suck's oil in past the valve stems whenever they had high manifold vacuum and if you look under the rocker cover straight after the engine has run, you will see because of the slope of the head that the oil lays around those valve stems.
Put a set of valve stem seals in first then take the bus for a long two hundred kilometre run to settle it down.
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