try thinner engine oil
Does the motor turn over strongly with the battery power?
If not get the battery tested.
When it starts and runs does it go as it should?
is there any smoke from the exhaust?
Black or blue coloured smoke could indicate wear or damage in the engine.
If all ok then check the HT leads to the spark plugs, check them for any damage or moisture; these leads carry high voltage to the plugs and if they short due to moisture etc then the plugs will not get the high voltage they need to spark.
So first check all the electrical supply to the plugs; high and low voltage, but start at the plugs and work back towards the ignition key.
Plug leads, coils etc.If its a well used car the ignition switch could be worn.
If you have a fat blue spark at spark plugs then check the fuel system
Dont quit they are great cars.
Ive got a diesel passat with 256000 miles and its still going strong!
Good luck from Richard