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light ticking noise in the valve train indicates valve lifters
lefters will lwak and will take time to fill on first start.
Lucas is good, but not for lifter problem, as lifter oppening is very small and thicker oil is slower to enter
As for low engine temperature- replace thermostat
thernostat that is not fully closing will cause this.
So in your opinion is this a fatal engine fault and could a light experienced mechanic tackle the lifter problem?
it is not fatal, some cars go for years, I do not say that this is the proper way to go, but you can go like this until ready to do it.
On this Engine you can Adjust Valve clearence using shims.