I'm running 5 40 fully synthetic oil on my 1990 carlsson and am woundering if it's the reason a have really noisy tappets? Also is it safe to switch from fully synthetic? as i was told once you use synthetic, anything else will damage the engine. What would be a good oil to use? i live in the neartheast of england so it can get quite cold (freezing in fact) and every now and then really hot.

Thanks for your time
DAVID

Use 10-30. Synthetic or refined doesn't matter.
SAAB recommends 10-30 because viscosity extenders reduce the oil's ability to resist coking.